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Why is the sentence "You must have drunk something" incorrect, but the sentence "You must have been drinking" correct?

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    You must have drunk something is correct, is that what you were thinking of?
    – Michael Harvey
    Sep 28 at 17:34
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    "You must have drink something" is wrong because drink should be a Past Participle - "You must have drunk something". OR a "Past Tense" verb, as in "It must be that you drank something".
    – FumbleFingers
    Sep 28 at 17:34

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To say that someone has had (alcoholic) drink after drink after drink, we use the perfect continuous, the continuous underscoring the idea of repetitive or ongoing behavior, and the perfect indicating the relevance of that past behavior to some point in the future relative to that past behavior. Since must doesn't have a past tense, we don't know the temporal context from your sentence alone:

I got pulled over by the police on the way home today.

-- You must have been drinking!

I got pulled over by the police a few years ago on my way home.

-- You must have been drinking!

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    Despite today / a few years ago, your examples don't represent Present / Past tense - they're both Past. I know it's a weak example, but Present Tense might be, say, "I'm worried I'll get pulled over by the police when I drive home", "You must be drinking" (if you weren't currently drinking, why else would you be worried?).
    – FumbleFingers
    Sep 28 at 19:03
  • must being modal doesn't convey anything temporal and so have been and the time-phrases "today" and "a few years ago" are the only time markers. The tense of have been is not simple past
    – TimR on some device
    Sep 28 at 19:09
  • The tense of have been isn't Simple Past (I never suggested otherwise), but it is Past. I addressed must as a past tense verb many years ago on ELU, but I would point out that not all modal verbs "lack" a Past Tense form. Consider can and could.
    – FumbleFingers
    Sep 28 at 19:16
  • Sorry, I am not understanding what you mean by "it is past". Not sure what the referent of "it" is. must as you know does not show any tense markers differentiating present necessity from past necessity.
    – TimR on some device
    Sep 28 at 20:25
  • the referent of "it" is The tense of have been, which I contend IS "past". It's irrelevant that have been happens to be used in the same utterance as must (which we obviously agree no longer inflects for tense, so mote it be! :)
    – FumbleFingers
    Sep 28 at 22:58
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You must have been drinking implies that the person being addressed is obviously under the influence of alcohol, so they must have been spending some time having repeated drinks.This is why the continuous tense is used.

You must have drunk something doesn't imply multiple alcoholic drinks.

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'must' is a modal verb and after modal verbs infinitives are used. There are 8 types of infinitives in English: https://simple.wikipedia.org/wiki/Infinitive

'must have drink' doesn't match any of these infinitives because 'drink' is not a past participle.

'must have been drinking' matches one of these infinitives. It's a perfect progressive infinitive (have been + ...ing)

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