Phrase structure rules for the auxiliary

The English verb thus has many potential auxiliary elements that must be accounted for in our phrase structure rules. Consider the following sentences:

1. John wrote a book. 4. John is writing a book.

2. John should write a book. 5. John is going to write a book.

3. John has written a book. 6. Write a book!

If we consider the auxiliary (AUX) as everything in the predicate but the verb phrase and cite the verb phrase in its uninflected base form, write a book, we see that the auxiliary + the verb write in each of the sentences above consists of the following elements:

1. past tense + write = wrote

2. modal should + write = should write

3. pres tense + perfect have... -en + write = has written (-en = past participle)

4. pres tense + progressive be... -ing+ write = is writing (-ing= present participle)

5. pres tense + the phrasal modal be going to + write = is going to write

6. imperative mood + write = write

As we previously mentioned, nonimperative English sentences obligatorily take gram­matical tense (-past, -pres) or a modal (e.g., will, can, must, shall, may). If some auxiliary verb other than a modal is present, it carries the tense. Beyond tense or a modal, three different optional auxiliary verb types may be present: a phrasal modal (e.g., be going to, have to, be able to); the perfect aspect (have + the past participle); and the progressive aspect (be + the present participle). If no tense-bearing auxiliary verb is present, the main verb will carry past or present tense (e.g., sentence I above).

Sometimes much more than tense or a modal auxiliary occurs in the AUX of a single sentence. For example:

1. John had to be writing a book. (AUX = past tense, phrasal modal, and progressive aspect)

2. John has been writing a book. (AUX = present tense, perfect and progressive aspects)

3. John should have written a book. (AUX = modal and perfect aspect)

4. John will have been writing a book. (AUX = modal, perfect and progressive aspects)

5. John will have to have written a book. (AUX = modal, phrasal modal, and perfect aspect)

6. John had to have been writing a book. (AUX = past tense, phrasal modal, perfect and progressive aspects)

Not all the combinatory possibilities are illustrated here. We discuss others in subsequent chapters. The tree diagram for the 6th example sentence above is:

As we have just seen, whenever three or more auxiliary elements occur together, the perfect precedes the progressive, and a phrasal modal precedes either of the two aspects. A modal can precede a phrasal modal and also either of the two aspects. If two or more potentially tense-bearing auxiliary verbs are present, only the first of these auxiliary verbs will carry the tense.

These relationships can be summed up in the following rule:

Here the auxiliary is AUX. It is made up of either -imper (imperative mood) or tense (T) or a modal (M), if (T) or (M) is selected; it can be followed by the other optional auxiliary elements: phrasal modal (pm) and the perfect (perf) and progressive (prog) aspects. (Imperative mood is a tenseless verb form in English.)

You have already learned that morphological tense in English is either past or present. This choice is stated in phrase structure rule 12:

The perfect and progressive aspects are expanded into their auxiliary verbs and accompa­nying grammatical inflections in rules 13 and 14 respectively.

We have been asked why we have a separate node for the auxiliary elements in our phrase structure rules for English. We do this because the auxiliary is syntactically very important in forming interrogative and negative sentences. It also carries tense, mood, modality, and voice and accounts for most of the morphological complexity in English. The subject and verb phrase, on the other hand, account for the propositional content of sentences.

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