We use modal verbs (must, may, could, might, can't) to show how certain we are about something.We use 'must' when we are almost certain about something - 'must + infinitive without to':That must be Jack. He always rings at this time.The past of 'must' is 'must have + past participle':He must have finished work by now. He must be on his way home.'May' is used to say that something is a possibility - 'may + infinitive without to':I'm going to fill the bath. Jack may want a bath when he arrives. He sometimes does.The past of 'may' is 'may have + past participle':He may have had a shower before he left work.'Could' is like 'may':I suppose he could have spent the day in the office, so he may not want a bath.'Might' is used to say something is possible but we think it's unlikely - 'might + infinitive without to':Of course, he might stop at the pub for a drink on his way home. He sometimes does, but he didn't say so on the phone.The past of 'might' is 'might have done / been':He's very late. He might have had an accident, although he's a very careful driver.We use 'can't' to say we think something is impossible and we have evidence for thinking this. 'Can't' is the negative of 'must' - 'can't + infinitive without to':He can't be at the cinema. He never goes to the cinema without taking me.The past of 'can't' is 'can't have + past participle':He can't have gone with someone else. He wouldn't dare!