may, might and could have

• We use may, might and could have + past participle for speculating about the past.

Maria isn’t here; she could have gone to the shops.

• We can also use the negative forms may / might not + infinitive and may / might not have + past participle.

However, we cannot use the negative form couldn’t have + past participle in this sense.

I’ve sent her a postcard, but it may not / might not arrive.