·The modals that get confused
Modals of deduction and obligation in English. With the same verbs, must, can't, might, should, we say very different things depending on whether they are followed by the infinitive or by have plus participle. We will see how to deduce something about the present, how to deduce something about the past, the difference between needn't have and didn't need to, and should have to criticise or regret. And a pitfall: must and have to do not always say the same thing. The rules are AptisBot's own criteria.
1/4Deducing about the present
To deduce about the present, the modal depends on how sure you are. Must: you are almost sure it is true. He must be tired; he's been working all day. Can't: you are almost sure it is not true. That can't be true. Might, may or could: it is possible, but not certain. She might be at home. Notice that to say something is certainly not possible you do not use mustn't, but can't. He mustn't be tired does not express a deduction. And remember that the infinitive without to follows.
2/4Deducing about the past
To deduce about the past, the same modals, but followed by have plus participle. Must have: you are almost sure it happened. The lights are off; they must have gone out. Can't have: you are almost sure it did not happen. She can't have seen me; she didn't wave. Might have, may have or could have: it is possible that it happened. He might have missed the bus. And watch out for the negative: could have is not the same as can't have, because could have is possible and can't have is impossible.
3/4Needn't have and didn't need to
Needn't have and didn't need to. They seem the same, but they are not. Needn't have plus participle: you did something, and it was not necessary. You needn't have bought flowers; the hotel had provided some. You bought the flowers, and it was not necessary. Didn't need to plus infinitive: it was not necessary, and we do not know whether you did it. I didn't need to buy flowers, so I didn't. The difference lies in whether you did it or not. And do not confuse the absence of obligation with prohibition: don't have to says it is not compulsory; mustn't says it is forbidden.
4/4Should have: criticism and regret
Should have plus participle is used to criticise or to regret something that happened. You should have called me. That is, you did not call me, and it was the right thing to do. With the first person, it is a regret: I should have studied more. And shouldn't have plus participle says that what was done was wrong: I shouldn't have said that. Ought to have is similar, but less frequent. And if you are only giving advice about the present or the future, you use should plus infinitive: you should see a doctor.
·To practise
To practise, look at something that happened near you today and write six sentences. Two deducing about the present, with must or can't. Two deducing about the past, with must have or might have. And two with should have or needn't have. Then check in each one what comes after the modal: the infinitive without to, or have plus participle.