Modal verbs

Modal verbs to express certainty and possibility

Present

Modal verb + infinitive

must

We use must when we are 100% sure about something.

She must be rich.

can't/couldn't

We use can't or couldn't in negative sentences.

Se can't be poor.

may, might, could

We use may, might or could to express a possibility about the present or the future.

She may have rich parents.

She might have rich parents.

She could have rich parents.

may not, might not

We use may not or might not for negative sentences. We don't use couldn't or can't.

She may not have rich parents.

She might not have rich parents.

Past

Modal verb + have + past participle

must have + past participle

We use must have + past participle to express certainty about the past.

She must have been rich.

can't have + past participle

We use can't have + past participle to express certainty about the past in negative sentences.

She can't have been rich.

may/might/could have + past participle

We use may/might/could have + past participle to express possibility about the past.

She may have had rich parents.

She might have had rich parents.

She could have had rich parents.

may/might not have + past participle

We use may/might not have + past participle to express possibility about the past.

She may not have had rich parents.

She might not have had rich parents.