In our last note on grammar, "A grammatical take on the future", we discussed how we indicate that an action is expected to happen in the future. This involved us venturing into the curious world of modality, which is concerned with assertions of probability, possibility, permission, intention, obligation and the like (4.2.2). We discovered that modality is expressed by placing a 'modal auxiliary' or 'modal' immediately before the main verb. There are five modals: 'can' (past tense 'could'), 'must' ('must'), 'shall' ('should'), 'may' ('might') and 'will' ('would'). We saw that modality is also expressed using 'semi-auxiliaries', such as 'be going to' and 'have to', in the same way.

Before we can go into more detail about auxiliary verbs, we need to backtrack and look at main verbs – their characteristics and the role they play in a sentence. Because auxiliary verbs (or helping verbs) work alongside main verbs, it is not possible to understand one without understanding the other.

In our article "Appearing for sentence", we looked at the elements of a sentence and explained that a sentence is made up of a subject and a predicate. The subject is the person or thing performing the action that the sentence is talking about. Everything else in the sentence is the predicate; for example:

I love you madly.

In this sentence, I is the subject because I am the person doing the loving, and love you madly is the predicate. If you look at the predicate, you will see that it begins with a verb – in this case, the verb love. All sentences must include a verb, because the verb tells us what action the subject is performing.

Many sentences, but not all, contain an object – a person or thing that is being acted upon. In this case, the object is you. And many sentences also contain other information, presented in various types of grammatical phrases – in this case, the word madly is an adverbial phrase. But objects and adverbial phrases are not needed to make a sentence and we won’t cover them here.

Main verbs can take five forms, which indicate things like tense and aspect. They are the base, present, present participle, past and past participle (7.2):

Base: laugh
Present (-s): laughs
Present participle (-ing): laughing
Past: laughed
Past participle (-ed): laughed

We saw in previous notes that the present tense uses the base and present forms, and the past tense uses the past form:

Simple present: I, you, we and they laugh; he or she laughs
Simple past: I, you, he, she, we and they laughed

The participle forms are used to indicate aspect, in conjunction with the auxiliary verbs be or have:

Progressive aspect: I am laughing
Perfect aspect: I have laughed

This pattern of forms, in which the past and the past participle is the same, applies to all regular verbs. (Apart from the verb 'to be', which is highly irregular, irregular verbs tend to differ in the past and the past participle; for example, ‘write’ has ‘wrote’ for the past and ‘written’ for the past participle.)

The form the verb is in tells us whether it is finite or non-finite. A verb is finite if it displays tense and non-finite if it does not. We know that the present tense uses the base and present forms and the past tense uses the past form. Therefore, the base, present and past forms are all finite forms of the verb.

The non-finite forms are the present participle, past participle and infinitive. The infinitive is a form of the verb that does not show person, tense or aspect. It usually takes the form 'to laugh' and 'to write', but the 'to' is not always present. For example, the sentence I must write uses the infinite of the verb 'to write'. Note that the infinitive uses the base form of the verb.

The issue of finite and non-finite verbs is crucial because it takes us to the last requirement of a sentence – it must have a finite verb; that is, it must show tense. Phrases that do not contain a finite verb leave us hanging; for example:

Writing the book (present participle)
Written the book (past participle)
To write the book (infinitive)

Even by adding a subject, you cannot make a meaningful sentence; for example: You writing the book. Contrast these with the verb forms that are finite:

You write the book or Write the book! (base form)
She writes the book. (present form)
She wrote the book. (past form)

In the next note on grammar, we will look more at the role verbs play in sentences and begin to examine the differences between main verbs and auxiliary verbs.

Reference: Sidney Greenbaum: The Oxford Reference Grammar, edited by Edmond Weiner, Oxford University Press, 2000.