Can we use the must have with 'I guess' for the past actions?

Example: I guess you must have watched that video twice.

Is 'I guess' valid here or should we replace it with 'I believe'?.

  • The sentence has redundancy. I suggest "I guess you have watched that video twice" or "You must have watched that video twice". Feb 13, 2018 at 8:31
  • 3
    @WeatherVane: there is no redundancy at all. These two terms express meanings in completely different domains.
    – JavaLatte
    Feb 13, 2018 at 8:59

3 Answers 3


Yes, it's perfectly normal to use "I guess" with "must have". The former indicates that you are expressing a loosely-held personal opinion ("to form an opinion of from little or no evidence" according to Merriam-Webster): the latter indicates that the opinion concerns something that, based on the available evidence (which may be scant, especially if used with "I guess"), probably happened in the past.

“This his day off?”
“No. I guess he must have called in sick or something.” Fair Game- B.W. Battin

I guess is informal US english: You could also use I suppose, which is common in British and American english. I believe is also possible, but it sounds quite formal in both American and British english. This NGRAM shows the relative frequencies of use.


You can certainly say I guess in this situation. The only thing to worry about is what exactly you're trying to say with that statement. We use the construction must have when we have a pretty strong reason or evidence to believe that something is true, but we're not one hundred percent sure whether it's actually true. For instance:

It's funny that you remember so much. Ha ha. I guess you must have watched that stupid video twice.

Here, the fact that the person I'm talking to remembers so much from the video suggests that they might have watched the video not just once, but twice. Although we're not one hundred percent sure whether that's true, what we can gather from the available evidence allows us to come up with a pretty good guess that it may in fact be true that they watched the video not just one time, but two times. So, I guess, in this particular case, actually sounds like a better choice of words than I believe.

I believe is very similar to the expression I think, to the point that in everyday conversation they can be sometimes used interchangeably. When you say that you think or believe that something is true, it's not a wild guess. Quite the contrary, there is an element of certainty in what you're saying. That's why I believe wouldn't work as well as I guess in this situation.

To sum things up, a lot depends on context. The devil is in the details, as they say. You just always need to be a little bit more precise with whatever it is that you're trying to say.

  • I disagree that believe and think can be used interchangeably. Yes, the meaning can be the same, but believe is a lot more formal: it is rarely used with a that-clause in everyday conversation. Nobody says "I believe she's hot!" in a bar on a Saturday night: they say "I think she's hot!".
    – JavaLatte
    Feb 13, 2018 at 10:40
  • I don't see any difference between "I think he's right" and "I believe he's right". Feb 13, 2018 at 10:55
  • When expressing an opinion, there is no difference in meaning, but there is a difference in frequency of usage, especially in informal usage (everyday conversation). Even in written english, which is biased toward formal or period usage, the split is quite noticeable: books.google.com/ngrams/…
    – JavaLatte
    Feb 13, 2018 at 11:11
  • 2
    @CookieMonster To me the latter expresses a far greater certainty and a kind of absolutism, and I would use one or the other based on that. Feb 13, 2018 at 14:30

Using "I guess" implies a certain percentage of possibility whereupon with "must have" it refers to the possibility of a probability.

  • You must have put the papers elsewhere. - probably you put them elsewhere.
  • I guess that you put the papers elsewhere. - I think that it is possible that you put the papers elsewhere.


  • My guess is (I guess) you must have put the papers elsewhere. - you probably put the papers elsewhere and I think it is possible that it is true.

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