Sentence Structure

Subject /Modal verb /Neg. form /Main verb /Object

She / be /must / very happynot /

Could , Might, MayAre used to speculate about something the speaker or writer is unsure about.

That star you’re looking at might in fact be Jupiter.

May is sometimes a little bit "more sure" (50% chance); whereas might expresses

more doubt (maybe only a 30% chance).

Must, may, might

Imagine that we have three keys on a ring and we know that one of these keys opens the trunk.We pick up one of the keys and say:

But after trying two keys unsuccesfully, we will pick up the third key and say:

This may/might be the key

This must be the key!( no other

Choice remains)

May is more certain than might

Must haveIs used to express near certainty about something in the past.

Past action or situation: must+ perfect infinitive

I rang your flat yesterday. A woman answered

But I didn’t recognize her voice.

Oh, it must have been my sister, Ann. I don’t think you have met her

yet..

Can’t / Cannot Couldn’t / Could notAre also used to indicate certainty, in relation to impossible ideas and situations:

A man answered the phone. I suppose it was her husband.

It can’t/couldn’tHave been her

husband. He’s been dead for 2 years!

Couldn’t / Could notCan also be used in questions, sometimes with possibly, to speculate about something:

You can’t /couldn’t be hungry. You’ve just had

dinner.

Can I have some sweets, mom. I’m

hungry!

Couldn’t is used to indicate an

impossible situation

Could have, Might have, May haveAre used to express uncertainty about something in the past:

I think I may have met you before.

The dinosaurus might have survived without the meteor impact.

Couldn’t have / Can’t haveAre used to express certainty that something in the past was impossible or didn’t happen:

He couldn’t have damaged your bike – He was with me all evening.

It can’t have been raining, as the path is completely dry.

Choose one of the words from the box below to fill in the gap.COULD MAY MIGHT CAN MUST CAN’T

1. You’ve been playing tennis in this hot sun for almost four hours. You…………… be thirsty!MUST

CAN’T

CAN

COULD / MIGHT / MAY

2. Surely she……….. know about the secret holiday we’ve planned. We’ve told nobody.

3. It………… get quite noisy in this classroom when everyone arrives in the morning.

4. I feel very tired. I think I……………………………….be getting a cold.

CAN

• Complete the sentences. Circle on the correct word.1. Jack ............. be at home. His jacket is still here.a) Might b) can‘t c) could

2. This .......... be the right answer. The others are definitely wrong.

a) can‘t b) must c) could

3. With luck, tomorrow ……………… be a sunny day.

a) could b) can c) couldn’t

CAN’T

MUST

COULD

Examples and explanations taken from A Practical English Grammar,A. J. Thomson and A. V. Martinet, Oxford University Press. Cambridge Objective

First Certificate , Annette Capel, and Wendy Sharp.