Where to use "must/should/might/may/could/shouldn't have "in a sentence?

Question

Where to use "must/should/might/may/could/shouldn't have "in a sentence?

We had in our book that "must+perfect infinitive" is used when we feel sure about sth in the past.And "might/may/could+perfect infinitive" should used when we think sth was possible but we aren't sure.And we should use "should+perfect infinitive" when we talk about past events that did not happen.

Now we had exercises about choosing them.Now I have some question.

1.How can we decide which one is "may/might/could+perfect infinitive" when they are the same?

2.We had an exercise "Nobody answered the phone at the clinic.It................ closed early"

Here the answer written is "must have" instead of being probable we aren't sure then why did we use it?

And also we had "Sarah looks really pleased with herself.She ...............passed her driving test this morning" which gives the answer "must have"but we aren't sure.

Answer 1

1. There is no clear answer to how to choose among "may", "might" and "could". There is really no difference between them in this sense; (though they all have other meanings as well, that in speech would be quite clearly distinguished by tone of voice, but in writing might be ambiguous.)

2. Your book is over-simplifying, in stating that the difference is in whether you are sure or not. You don't have to be absolutely certain to use "must": rather that it is the obvious deduction, or the likely reason. With the other words it is one possible conclusion or reason, but other possibilities are just as likely.

So, if "nobody answered the phone at the clinic", there are other possible explanations: maybe the phone system is broken, or everybody there has fallen asleep. Maybe it's closed all day because they're on holiday. But you say "it must have closed early" to mean "I deduce from what I know that it has closed early).

Similarly with Sarah. Perhaps she's won the lottery, or somebody's just proposed to her. But if you knew that she was taking her driving test today, and saw her looking really pleased, you would probably deduce that she had passed it. So you would say "she must have passed it". On the other hand, if you knew that she had her test coming up, but didn't know whether it had happened yet, you might say "She could have passed her driving test". (Or you might leap to the conclusion that she has had her test and passed it, and say "must have").

Answer 2

In the second example you are "making a deduction" that becuase nobody answered the phone, then it is "very likely" that the clinic must be closed.