In phonetics, the phoneme was the basic constituent. On a phonetic account, all things boil down to concatenations of phonemes. And on top of that lies phonology, the basic constituent of which is the syllable.
What is the basic unit of syntax? The word. But what determines the boundaries of the word? It is clear it isn’t phonological: syllables may make up words, but words are not defined by syllables; they can be one, two, three or more. And phonetics will give us no answer. A glance at the soundwaves of speech will show that the space in between words is not a phonetic gap. Morphology is no help either. Words such as employ-er or waste-basket are made up of multiple morphemes.
There may be temptation to define a word orthographically. A word is what has no letters on either side of it. But then why is their ambiguity about words such as swimsuit/swim-suit/swim suit? For now, I will define words rather arcanely: they are the members of syntactic categories.
The secondary constituents of syntax are phrases, or the combination of any two words. For example in the sentence
(11) The Duke of Earl will brush his teeth in the airplane
the words do not simply combine to make a sentence; there is a sense in which some words are closer than others.
That is, the proper grouping of (11) is not
(11b) [S [Det The] [N Duke] [P of] [N Earl] [Aux will] [V brush] [Det his] [N teeth] [P in] [Det the] [N airplane]]
Firstly, because this yields rules which are too specific: they shan’t generalize to other sentences.
S –> Det + N + P + N + Aux + V + Det + N + P + Det + N
Det –> The, his
N –> Duke, Earl, teeth, airplane
P –> of, in
Aux –> will
V –> brush
How do we determine what the constituents are? First, our intuitions will play a major role. For example, it simply seems intuitive that his is bound to teeth rather than to brush, and that [his teeth] is a constituent, but [brush his] is not. But there may be unclear cases, and so we should develop tests to detect constituents. The first test is Movement: since constituents are bound together, one should expect only constituents should be able to move. Compare the following sentences
(11c) his teeth, The Duke of Earl will brush
(11d) *brush his, The Duke of Earl will teeth
While (11c) may be stilted, it is clearly a better sentence than (11d). Another type of constituency test comes from question formation. Wh- questions can only form from constituents
(11e) What will the Duke of Earl brush in the airplane? His teeth
(11f) Whose teeth will the Duke of Earl brush in the airplane? His
(11g) Where will the Duke of Earl brush his teeth? In the airplane
(11h) Who will brush his teeth in the airplane? The Duke of Earl
(11i) What will the Duke of Earl do? Brush his teeth
It is hard to show an ungrammatical example, simply because there is no wh-word to express something like ‘eat his’. But since the Wh-word stands for a constituent, wh-movement indicates that the answer to the question must be a constituent. I will save the guesswork of figuring all of the constituents out, and present the following as a rough account of the above sentence (I have left ‘of Earl’ out for the purpose of simplification)
(11j) [S [NP [Det The] [N Duke]] [Aux will] [VP [V brush] [NP [Det his] [N teeth]] [PP [P in] [NP [Det the] [N airplane]]]]]
Giving us the phrase rules
S –> NP – AUX – VP
NP –> Det – N
VP –> V – NP – PP
PP –> P – NP
Det –> The, his
N –> Duke, teeth, airplane
P –> in
Aux –> will
V –> brush
With this set of rules, and our limited lexicon, we can already generate many grammatical (although mostly nonsensical) sentences, such as
(11k) The Duke will brush his airplane in his airplane
(11l) The teeth will brush his airplane in the Duke.
This may seem trivial, but it is the notion of syntactic categories, and the shuffling of words of the same syntactic category, which allow for a finite rules system to generate infinite sentences. Needless to say, this grammar, however, is incomplete. Even assuming lexicon expansions, it is not equipped to deal with sentences such as
(1) Sven kicked a puppy
(5) I gave Jeff the money
(9) Joey smokes Pall-Malls