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  • Writer's pictureTom Garside

Modal auxiliaries and auxiliary verbs… What’s the difference?

Auxiliaries are a major part of English grammar constructions. They appear in tenses, questions, negative forms and other functional structures which can change the meaning and grammar of a sentence significantly.

However, they can be a challenge as many auxiliaries look the same, but do a range of different jobs. So how can we clearly differentiate between different types of auxiliary?

Auxiliary verbs

Auxiliary verbs can be thought of as ‘helping verbs’. They work together with main verbs to ‘help’ them change their form. The three auxiliary verbs which are most commonly used are easy to remember: the verbs do, be and have, which I always remembered through the mnemonic ‘do behave!’.

Auxiliary verbs work like ‘grammar sponges’. They work with main verbs to absorb and show grammatical aspects of meaning such as time, number and person. In tenses, it is the auxiliary which agrees with the subject: I am, he is, they are, etc., where the ‘be’ auxiliary changes its grammar to form the present continuous, as in:

I am eating lunch

He is eating lunch

They are eating lunch

Notice that the main verb (the verb that the sentence is ‘about’) does not change its form - all of the grammar is ‘absorbed by the auxiliary. This is also true if we put these sentences into the past:

I was eating lunch

He was eating lunch

They were eating lunch

Again, the main verb remains unchanged as a present participle V+ing form in each sentence, and it is the auxiliary which shows the past-ness of the tense.

These three simple verbs also work with main verbs to make questions:

Do you like chocolate?

Are they interested in the environment?

Have you ever been to Italy?

and negative forms:

I don’t like chocolate.

They aren’t interested in the environment.

I haven’t been to Italy.

Again, the question is shown by the position of the auxiliary (before the subject) and the negative -n’t suffix attaches to the auxiliary. This is what is meant by ‘grammar sponge’.

Modal auxiliaries

Modal auxiliaries (or modal verbs) operate slightly differently form auxiliary verbs. They are also auxiliaries, but work in a more functional way. Modal auxiliaries (like can, will, must, shall) add a function to the main verb they work with, which is always used in its infinitive form (without any endings or other grammar).

Modal functions are tricky, but very teachable forms. They have present, past and perfect forms, but do not in themselves form tenses. Also, modal structures can have many different functions depending on how they are used, and individual functions can be communicated using many different modals. The structures and functions of modals represent a very complex relationship, so should be taught one by one rather than trying to combine various forms and meanings in one class (except at higher levels in review activities).

Time and modality

The present, past and perfect modals work as follows, with present and past modals being followed by infinitive verbs, and perfect modals being followed by a ‘have’ auxiliary, and a past participle (as in perfect tenses):

Present Past Perfect

can could could have + Vpp / can’t have + Vpp

may/might might might have + Vpp

will would would have + Vpp

shall should should have + Vpp

must must must have + Vpp

A good way of teaching modals (though perhaps not the first time they are introduced) is to take one modal and brainstorm many of its functions, or take one function and explore the different modals which can communicate it, as in:

Modal Function Example

may polite request May I have some more, please?

possibility It may not work…

permission You may leave when you are finished.

Function Modal Example

Possibility can It can get cold in Spring

could He could be away from his computer

(Past) could have They could have been hurt

might It might rain later

may This answer may be correct

Taking modals or their functions one at a time breaks down this huge area of functional language, and makes it manageable for developing learners. Try spending 30 minutes per week doing this, and your learners will find their functional language increases quickly to communicate with modals more clearly.

As we have seen, auxiliaries form a fundamental part of English grammar, and are a common area for student errors. An understanding of how they work in terms of placement in sentences, grammatical and communicative functions can help students to get a handle on auxiliaries, and will allow you to work with them more confidently in your teaching.

Tom Garside is Director of Language Point Teacher Education. Language Point delivers the internationally recognised RQF level 5 Trinity CertTESOL in an entirely online mode of study, and level 6 Trinity College Certificate for Practising Teachers, a contextually-informed teacher development qualification with specific courses which focus on online language education or online methodology.

If you are interested to know more about these qualifications, or you want take your teaching to a new level with our teacher development courses, contact us or see our course dates and fees for details.


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