Why do we use passive present perfect like in this sentence?
If the bribe were given, it must have been given to officials.
Can't we use passive simple past?
Also, why can't we use past perfect 'had been given' instead of 'have been given'?
The verb in your sentence is not the present perfect have been given.
The verb in your sentence is must have been given. You can call it the modal perfect passive.
Here the modal is must; the perfect is indicated by have; and the passive is indicated by been given.
A modal verb such as must (other modals include will, would, shall, can, could, may, ought, and dare) is followed by the bare infinitive. It cannot be followed by a verb that is finite.
The has in the present perfect passive it has been given is a finite verb, conjugated for tense (present), person (third), and number (singular).
The have in the modal perfect passive it must have been given is non-finite, and does not indicate tense, person, or number.
In standard English, the following forms do not occur:
*must had been (given)
*must has been (given)
The modal perfect must have refers to a completed action. Thus a speaker can use it to express 100% certainty about a past occurrence. The same is true for the modal perfect passive must have been.
Thus it must have been given to officials expresses the speaker's certainty that if a bribe was given, the speaker is 100% certain that the bribe was given to officials.
In short, we use must have (active construction) and must have been (passive construction) to talk about our certainty concerning a past action or state. Note that this certainty of the speaker does not have to actually be what happened.
It must have rained.
I'm 100% certain in my belief that it rained.
I thought it must have rained, but it didn't.
I was 100% certain that it rained, but in fact it didn't actually rain.
For more information on modals and how other modals express different amounts of certainty or possibility, see the Modal Tutorial at English Page dot com.
There is a very important rule that you have to follow in English. A "bare infinitive" or "base form of verb" should follow all the modal verbs such as "must". That's why "had" can't follow "must" as it is the past form of to "have".
Now, I am not sure what you mean by "passive simple past", but if you mean "must had been given" by it, it is plainly wrong. It should be "must have been given".
"Must have been given" is not in the present perfect tense. It is in the past tense and it is used when we are very sure about what happened in the past.
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