Grammar Reference. Unit 9

9.0 Auxiliary verbs. Modal verbs.

Modal verbs can express ability, obligation, permission, and request. They can also express the idea of probability or how certain situation is. Here is an introduction to modal auxiliary verbs

Auxiliary verbs

Auxiliary verbs are small verbs used to build verb chains.

Kate is speaking.
We had been asking.
She did like him once.

The auxiliary verbs are:

• have when followed by a past participle:
  • e.g. have seen
• be when followed by a present participle or by a past participle:
  • are working
  • are admired
• do when followed by an infinitive:
  • do you think, do not think.
• the modal verbs, which are generally followed by an infinitive:
  • will think, must be

Modal verbs

Modal verbs are auxiliary verbs that come in front of an infinitive (usually without to). They express such ideas as possibility, willingness, prediction, speculation, deduction, necessity and habit.

He must be angry
I can’t understand.
You ought to tell us.

Here are the main modal verbs:

Why be is always an auxiliary verb and possessive have sometimes is.

One of the main differences between auxiliary and main verbs is that auxiliaries are used in questions like Are you listening? and in negative sentences like You aren't listening. If we apply this as a test for auxiliary verbs, then other uses of be should also count as auxiliaries:

The same applies, for some people, to the verb have which means 'possess':

Modal verbs of probability

9.1 Expressing possibility / probability : The present / future

1. Must and can't express the logical conclusion of a situation:
must = logically probable
can't = logically improbable

We don't have all facts, so we are not absolutely sure, but we are pretty certain.

He must be exhausted. He can't even stand up.
Sue can't have a ten-years-old daughter! She's only 24!
He's in great shape, even though he must be at least 60!
A walk in this weather! You must be joking!
Is there no answer? They must be in bed. They can't be out this late!

2. Could and may/might express possibility in the present or future.
May / Might + not   is the negative.
Couldn't is rare in this use.

He might be lost.
They could move to a different place.
Dave and Beth aren't at home. They could be at the concert, I suppose.
We may go to Greece for our vacation. We haven't decided yet.
Take your umbrella. It might rain later.
I might not be able to come tonight. I might have to work late.

The continuous infinitive
Must / could / can't / might + be + -ing make the continuous form in the present.

Peter might be working late.
They can't be working very hard.

Compare:

"John's grass is lovely. He must cut it regularly." (habit)
"What's John doing in the garden?" "He might be cutting the grass." (now)