Should vs. must

[Ken]

Test No. incompl/elem-27 "Sailing", question 9

That ......... have been horrible. So what are you doing this weekend?

(a) must
(b) may
(c) can
(d) should

Test No. incompl/elem-27 "Sailing", answer 9

That must have been horrible. So what are you doing this weekend?

Correct answer: (a) must

Your answer was: incorrect
That should have been horrible. So what are you doing this weekend?

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i tought 'must' can be used only in present sentence

[Alan]

Must cannot express tense and so the idea of Present Perfect is expressed in the infinitive: have been.
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English as a Second Language
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[Ken]

Sorry , but i don't understand you . If 'must' cannot express tense , why we must use it in this case .
Thks You

[Alan]

Let me try again:

I must be late because the room is now full of people. This means it is clear that I am late.

I must have been late because the room was full of people. This means it was clear that I was late.

It must be Sunday because all the shops are closed. It means it is obvious it's Sunday.

Must in these examples simply means it is clear/obvious/ definite.

Must can also be used to express it is necessary. I must leave early in the morning to catch my train. This means it is necessary for me to leave early.

In both cases verbs after must are in the infinitive and it is that part of the verb that indicates the time or tense. Must is a so-called defective verb because there is only one form and it has no principal parts. If you want to express a time, you use alternative verbs as : I must leave now and in the past you use the alternative have and write: I had to leave then.
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[Ken]

Ok thks you ,
that's clear now .