Grammar Notes

Should - to express obligation

Structure: should + infinitive

We use should for the present and the future.

We use should to give advice to someone and to say that something is a good idea.

Should is weaker than have to or must.

You should tell them the truth.
You shouldn't smoke, it's bad for you.
I don't think you should do it.

Might - to express probability

Structure: might + infinitive

We use might for the present and the future.

We use might to express uncertainty (not sure) about events or facts .

Should is weaker than have to or must.

He might be telling them the truth.
They might not come on time.
That might be true, I'm not sure.

Negation

Positive:

must (used for a personal opinion)

He must be crazy.

Negative

can't

He can't be crazy.

Positive:

must / have to / has to (used for obligation)

You must arrive by 8 o'clock.

You have to arrive by 8 o'clock.

Negative

don't have to / needn't = something is not necessary

You don't have to arrive by 8 o'clock.

You needn't arrive by 8 o'clock.

Positive:

can (used for ability)

I can play the guitar.

Negative

can't / cannot

I can't play the guitar.

Positive:

can (used for possibility)

You can smoke here.

Negative:

can't / cannot / mustn't

You can't smoke here.

You mustn't smoke here.