Sentence structure and Clauses

[category: Linguistics]

Grammar and Syntax

Grammar and Syntax are not the same thing, but they are very closely related. Syntax and Morphology (or ‘Morpho-Syntax’, even) are the sciences of how the mind interprets and produces language; Syntax is mainly concerned with sentence structure, not the form of the words (the realm of Morphology). Grammar is the name we give to the rules within each culture that describe what we consider to be the most proper use and ordering of words. However, unsurprisingly, given that language is a natural, not man-made, phenomenon, formal grammar in English corresponds closely to the most Syntactically natural ordering of words.

When the  word-ordering in a language differs from what is most Syntactically natural, it creates tension (i.e. a small amount of extra mental processing is required), which gives one word or phrase more emphasis than others. The most Syntactically natural word-ordering, with the least tension, is not necessarily the most common or natural in everyday speech, because conversational speech involves a lot of emphasis and many stylistic effects – which make use of tension. The most Syntactically natural form of English, with the lowest cognitive tension, is formal English.

To create tension, the speaker can re-order words, and the brain accepts certain re-orderings; but some it cannot process, or they’re just too arduous to process, and they sound ungrammatical. There is a certain grey-area, a point at which the speaker can break the rules, which causes a ‘double-take’, an extra amount of processing by the hearer, but the hearer’s mind still accepts the speech as meaningful and grammatical.

Linguists are often condescending of Grammarians, but I believe you can think of English formal grammar as (among other things) the empirical science of the natural ordering of English Syntax.

Recap and basics

I explained some of the basics of Syntax diagrams and  how Linguists believe the mind structures language in the post on X-bar theory.

That post contained two points relevant to this one:

1) That language is structured into trees of small phrases (known as XPs), each of which essentially has three positions:

  • a Spec, which can (but doesn’t have to) contain a phrase, or XP, which often moves there from somewhere else. This XP is often from further down the tree (for example, as we will see below, a pronoun moving to the clause-initial position in a relative clause), in other words, another XP.
  • a Head, which contains a single element, usually a single words.The Head position is where the active constituent of the XP appears. In a Noun Phrase, the Head is a noun, in a simple Verb Phrase, it is a verb, and so on.
  • a Complement, on the right which can contain a ‘daughter’ XP. The entire tree is made of linked XPs, each (prior to movement) a Complement of the element before it – a Noun may be the Complement of an Adjective or Article; the Object may be the Complement of a Verb in a simple Verb Phrase, and so on.

Here’s a simplified representation of such an XP. This is not how XPs diagrams are drawn in Linguistics – in Syntax, researchers are concerned with the details of movement, ‘Command’ and ‘Governance’, but in these posts I’m interested in sentence structures, so this is a simplified diagram such as would not be used in a Linguistics research paper.

Simplified XP


2) The basic XPs that make up an English sentence or clause are as follows:

Sentence and clause - tree structure

In simplified (non-standard) form, clause/sentence structure has the following phrases, or positions.

simplified sentence

The English Verb Phrase (or VP, as Linguists call it) is complicated. It actually consists of a number of XPs, with automatic movement of elements within the Verb Phrase, so the structure is not just a simple XP. Adverbs each have their own XP, and there can be strings of them. I will look at the Verb Phrase and Adverbs in a different post. In this post I am interested in the top of the clause, the bits on the left: the CP and IP – the Complementiser Phrase and the Inflect Phrase.

The English sentence

The Inflect Phrase (the IP) is the heart of the sentence. In English, in a normal statement, the IP is the top of the sentence. It always contains the subject, in its Spec position, which never moves. The Head of the IP also contains the first Auxiliary Verb or Modal Verb (if the clause has one), but it never contains the Main Verb*. The Head of the Inflect Phrase is where inflection occurs (inflection is where verbs ‘agree with’ their subjects, e.g. “he has done” “they have done”).

[* the Main Verb  (or Lexical Verb, in Linguistics) is the verb that carries the meaning. For example: “she has left“, “she departed – the bold-italic verb is the Main or Lexical Verb”].

The Complementiser Phrase (the CP) is where things happen to make a sentence something other than a simple statement. As we’ll see below, it’s where questions are formed, and it’s also the part that connects a Clause onto another XP, in other words, that makes it a Subordinate Clause (or at least, certain types of Subordinate Clauses). Finally, a process called Topicalisation can occur, where elements from the sentence move into the CP for emphasis.


In ordinary speech, the most common Syntactic process that uses the CP is the formation of questions. The simplest way for this to happen is for the Auxiliary or Modal Verb to move from the Head of the IP to the Head of the CP (Head-to-Head movement, explained here), in the first example below. In grammar this is called an inversion, but that isn’t accurate – the subject of the sentence in English is very stable: it almost always appears and it seldom moves – in questions, only the Verb is moving.

Alternatively, an Interrogative Pronoun (the Question Word, as we say in English teaching, or a Wh-word, as Linguists call them), can appear in the Spec of the CP. What actually happens is that one part of the sentence is replaced by a pronoun, that then moves to the CP, at the same time as the Auxiliary or Modal Verb moves.

sentence - question

Note the difference between the Head, that contains a single ‘active’ element, which can only move from a Head position in a neighbouring phrase, and the Spec position, which can accept a phrase from anywhere. In fact, Phrases can enter the Spec position from ‘out of nowhere’, which they do when there is no Auxiliary or Modal Verb to move, such as when:

We went
Did you go?

In Syntax, this is call do-insertion, and it occurs because the Main Verb is unable to leave the Verb Phrase.

A pronoun standing in for any part of a statement can appear in the Spec of CP. The Spec can even contain a phrase with two pronouns. Here are some examples:

sentence - question2

In each case, some part of the sentence is changing to a pronoun and moving to the Spec of CP; and a Modal or Auxiliary Verb is moving to (or appearing in) the Head of IP.


Sometimes, some elements in the Clause can move into the CP for emphasis. There are some interesting aspects to this, which will be discussed in a separate post.

My Mother I gave a box of chocolates and my father I gave a bottle of wine.

This is an extremely common place to find Prepositional Phrases and other Adverbials, although – as will be discussed elsewhere – some of these may actually lie outside the Clause.

Here are some examples:


There are all kinds of Topicalisations, and these are just a few. I believe that certain Adverbials (such as the Prepositional Clause in the second example above) are not topicalised, but lie outside the Clause, but that is a topic for another post.

Complement Clauses (such as in Reported Speech)

In Syntax, anything that is located in the Complement position of the XP (above) – in other words, usually immediately to the right of it in English Syntax – is called a Complement; so the Complement of a simple Verb Phrase may be a Noun Phrase, the complement of an Adverb Phrase might be an Adjective  Phrase, and so on; but in grammar we use the word  Complement Clause to describe a clause (with a verb of some sort) that is the Complement of another verb. The type of Complement Clauses discussed here are finite Complement Clauses, because they contain a tensed (or finite) Verb, not an Infinitive or Participle (the other types of Verb complements). Finite Complement Clauses are most common in what, in English teaching, we call Reported Speech.

For example, in the sentence:
She explained everything,
“everything” (a noun) is the Object of the Verb.

Whereas in the sentence:
She explained that she was leaving.
“that she was leaving” is the Complement of the Verb, and is a Finite Complement Clause, which in this post I will just refer to as a Complement Clause.

A Complement Clause typically has a Pronoun in the Spec position of the CP; and it can’t have any content in the Head position. The exact same type of Clause can act as the Subject of a sentence (the first example below), or the Complement, the equivalent of the Object.

I tell my students that – in grammar terms – this process (placing the pronoun at the start of the Clause) Nominalises the Clause – in other words it turns it into a Noun Phrase. It isn’t really a Noun Phrase, but a Complement Clause can do most of the things a Noun Phrase can do (act as the subject of sentence, for example), so it’s a useful analogy.

complement clause

In a Complement Clause, the pronoun may replace another element in the clause, or it may (first example above, with the pronoun that), simply connect the whole clause, with all its constituents, to the verb.

The pronoun that is not required, of course, in many sentences. All English speakers and above-Intermediate learners know that you can usually leave out that with verbs of speech. In addition, however, it’s important to note that that nothing can be in the Head position of a Complement Clause, the reason for which I will explain below.

Relative Clauses

Relative Clauses also typically have a Pronoun in the Spec position, but also often don’t require them; and they also cannot have anything in the Head position. In Relative Clauses, the pronoun is formed by removing an element from somewhere else in the Clause.

relative clause1

In fact, Relative Clauses are formed in exactly the same way as Complement Clauses. There are some variations in usage and construction that apply to each type, but the the main real difference is simply that Complement Clauses attach to Verbs and Relative Clauses attach to Nouns.

She told me where she is living.
(Complement Clause, attaching to a Verb)

That’s the town where she is living.
(Relative Clause, attaching to a Noun)

Because the Spec position of the CP is filled, certain things that are theoretically allowed in formal English grammar sound strange and not quite natural. For example (see below), if a speaker tries to Topicalise a Prepositional Phrase in a Relative Clause the position it wants to move to – the Spec of CP – is already filled. This construction doesn’t break the rules of grammar (in fact you can squeeze a lot into a Spec position, they are designed to be flexible), but it causes a lot of tension, and is in the ‘grey-area’ in terms of mental processing – in other words, it doesn’t sound completely natural and requires a ‘double-take’ in processing, but the brain still accepts it as grammatical. As a result, it often needs to be spoken with one or more short pauses.

relative clause2

Functional Content in the Head of CP

The only content that can go in the Head of CP is the Auxiliary Verb, moving there from the Head of IP to form a question. However, Linguists assert that there is a type of content called ‘Functional Content’, which is unspoken (or ‘not realised’ as we say in Linguistics) – basically a thought, which is not expressed with a word or words, that can occupy a Head position (since the Head position is a very ‘active’ position). If there is Functional Content in the Head, nothing else can move to occupy it – so question forms are not possible in Complement or Relative Clauses.  The question requires movement of the Auxiliary or Modal Verb which is blocked by this unspoken Functional content.

This is why constructions like:
            …the man whom met I in the park?
are not possible.

functional head

Thus in plain English, in a Relative Clause or Complement Clause, the Head of the CP is filled with the ‘thought’ of it being a Clause, so that position must remain empty of spoken content. This explains the question students sometimes ask – why a Relative Clause can never take the form of a question.

relative clause3

Note also: you can squeeze things into a Spec position (the initial phrase position of an XP), because evolution seems to have designed it to be flexible, and apart from holding content it doesn’t do anything active (you can miss things out, double them up, and so on); but the Head position is a Syntactically active position – it’s where things ‘happen’, and the rules of what can and can’t go there are not flexible – you either have the ‘right’ content in the Head for the Clause type (even if it’s unspoken content), or the Clause doesn’t make sense.


There are a great many variations within these Clause types, whcih I hope to discuss elsewhere, but questions, Topicalisation, Relative Clauses and Complement Clauses are the main Clause types in English that make use of the Complementiser Phrase (CP), the topmost phrase of an English sentence. There are only two positions in the CP, so the number of variations are limited – the main variations being:

  • what goes in the Spec position, if anything;
  • whether there is content in the Head position;
  • what kind of phrase the Clause is a complement of (i.e. a Noun Phrase, for the Relative Clause; or a Verb Phrase for the Complement Clause).