CAN AND MUST

INFINITIVES

This lesson can be summarized very briefly:

The infinitive of ‘can’ is ‘to be able to’, and

The infinitive of ‘must’ is ‘to have to’

The verb ‘to can’ exists, but it means putting things (like sardines or tomatoes) into cans. In American English it also means to dismiss a person from a job – ‘he was canned last week’. But these verbs have nothing to do with the auxiliary ‘can’. There is no verb  'to must’.

It is necessary to know these two infinitives because without them you will not be able to form other tenses.

You must have the infinitive and the name of the verb

(the infinitive without ‘to’) in mind for these two auxiliaries

as you would for any other verb.

The present tense of ‘can’ is: I can, you can, he can, she can, it can, we can, you can, they can. No changes and no problem.

When you use an auxiliary it is normally followed by the principal verb you want to use. Simply for example I use the verb  ‘to tell’ in all these tenses –

Present: I can tell you

Simple past: I could tell you

Observe that ‘can’ has a simple past tense: I could, you could, he could, she could, it could, we could, you could, they could (also no changes).

In English we have only one word for the past tense and for the auxiliary: both are ‘could’. For example:

Simple past: I could run for miles when I was a boy.

Auxiliary: I could help you if I had a map.

The other tenses are formed in the same way as other verbs. In addition to the present and simple past shown above, here are the other three essential tenses:

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Formal future:

Person + will + name of verb: I will be able to

Example, I will be able to tell you.

Informal future:

Person + present progressive of ‘to go’ + infinitive:

I’m going to be able to

Example, I’m going to be able to tell you.

Present perfect:

Person + conjugation of ‘to have’ + past participle.

I have been able to

Example, I have been able to tell you.

MUST does not have a past tense. The present tense is –

I must, you must, he must, she must, it must, we must, you must, they must.

Example, I must tell you this secret.

In English ‘I must’ is synonymous with ‘I have to’.

This is the key to the other tenses of ‘must’ – except for the present tense -

we must use ‘to have to’ -

Simple past: I had to

Example, I had to tell you.

Formal future: I will have to

Example, I will have to tell you later.

Informal future: I’m going to have to

Example, I’m going to have to tell you.

Present perfect: I have had to

Example, I have had to tell you.

So, there is no “I would can” or “I will must”.

When you know the two infinitives:

Can = to be able to

and

Must = to have to

you can follow the construction of a verb tense in the same way as for

‘to walk’ or ‘to talk’ or any other verb.

Please see: Verbs - Introduction

and            Five Starter Tenses

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