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Are these sentences interchangeable?

  1. I think I have left my keys at home.

  2. I must have left my keys at home.
    For example: If somebody asks us "where have you left your keys?" which answer would be correct to this question, 1 or 2? and why?

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    What does Cambridge dictionary tell you about the verb must? What does it mean in English? When you understand that, you'll be able to answer your own question! I think I have expresses uncertainty. What does I must have express? The two expressions are close in meaning, but not interchangeable. I think I left my keys at home means that I'm not sure where I left them. I must have left my keys at home means that I know where I left them. – P. E. Dant Sep 12 '16 at 8:35
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    @P.E.Dant - Cambridge says: We use must have + ed form to talk about deductions in the past. It always refers to deduction, not obligation. – Mehrdad Moshaver Sep 12 '16 at 8:45
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    @MehrdadMoshaver I think this question has potential. Please remember to include more details with every post, like your thoughts on the matter and any research you have done. Specifically, I recommend that include the text from the Cambridge dictionary in your post and explain why you think 2 is equivalent to 1. Also, have a look through past questions on must have. You might find useful information, if not the answer to your question. – Em. Sep 12 '16 at 9:06
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    Your first sentence is speculation. You are pretty sure, but not fully sure, that you left your keys at home. You think, which means: Think. v. : to believe that something is true, that a particular situation exists, that something will happen, etc.--Webster's. So you believe they are at home. To say this, you use a noun clause as a direct object of the verb "think": (that) I have left my keys at home. [To be continued in my next comment] – Arch Denton Sep 12 '16 at 9:38
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    In the second, using the helping verb must which means, —used to say that something is very likely--Webster's. in your verb phrase "must have left" is saying, "I likely have left my keys at home." Why is it likely? Why is it likely they are at home and nowhere else? Because there is a reason, of which in the first sentence there is no reason or inferred reason, being inferred using the word "must." [Con't] – Arch Denton Sep 12 '16 at 9:40
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We can consider the usage naively, in terms of the speaker's intent, and as perceived by the hearer.

Naively, the first is deduction (due to think) or speculation (due to the lack of explicit facts), while the second expresses certainty (due to must).

In terms of intent, the first expresses uncertainty and the second expresses a form of certainty. The following definitions support this.

I think so 1 a. used for saying that you are not completely certain about something: ‘Is James coming tonight?’ ‘I think so, but I’m not sure.’ - Macmillan Dictionary

Must 1 used for saying that you think something is probably true because nothing else seems possible: They must have got lost or they’d be here by now. - Macmillan Dictionary

However, the reasoning assumed for must is often weak or absent in conversational use of the term. When absent, this use borders on false bravado.

To the hearer, then, both can be perceived to be uncertain, but in different ways.

Nevertheless, the terms are not interchangeable because the intent and connotations they convey are different.

4

I think I left my keys at home.

I must have left my keys at home.

Neither of those phrases has an "exact" meaning. The first talks about what you think may be true. A thought can just pop into our heads. The second talks about what you have concluded to be true. Conclusions take at least a little bit of reasoning.

We use "must" after we have eliminated some possibilities, for example, they're not in your other pocket either, and not in your brief case, and you don't remember locking the front door on your way out of the house. You'd probably remember locking the door if you had done so, and so the keys must still be inside the house.

Either one of those statements would be a grammatical response to the question "Where have you left your keys?" So would "It's none of your business, dude, where I left my keys."

2

X must Y means for some reason, X believes that Y is true or had been true and X is unaware of any reason to the contrary.

Valid reasons include:

  • logical inevitability: *John turned left so he must be at Mike's house (Mike lives down that street.)

  • you did something or saw something previously and have received no evidence to the contrary since then: "My keys must be in my top drawer (You put them there earlier)"

  • something bad will happen and the speaker/writer is sure you want to avoid it: "You must pay your rent."

  • a really strong version of should: "You must come over and play this game sometime."

  • sometimes used to issue commands. "You must put my keys away next time!"

X think(s) Y just means that X believes Y is true or had been true, but is leaving open the possibility he/she may be wrong.

My keys must be in the car. (For example, I am remembering that I left them there)

I think my keys are in the car. (I'm not really sure where they are but I'm saying this is likely.)

The right answer depends on how confident you are that Y is true. Must means you are very confident, think means you are not completely confident.

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    I would say there's a third option with even more certainty: "My keys are in the car". And in fact, that's what I would use in your first case (if I remember that they're there, then I am sure that they are there). I feel the "must" case does include a slight lack of confidence, because you've arrived at the conclusion by eliminating alternatives but don't have any direct evidence for that case itself. (E.g. your keys aren't in your pocket or on the table, and you had them when you got in the car, so you can't think of any other place they could reasonably be.) – Andrzej Doyle Sep 12 '16 at 15:12
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    Yes, it's counterintuitive to native speakers, but must actually adds uncertainty. – snailboat Sep 13 '16 at 8:06
  • Interesting, not a way of looking at it that I ever considered before. – LawrenceC Sep 13 '16 at 12:45
  • Andrzej's comment is fascinating, and I've never seen it discussed here. It certainly could be incorporated in your answer. For clarity though @snailplane - it's the construction must have that adds uncertainty, especially in things like I must have left my keys somewhere. – P. E. Dant Sep 15 '16 at 21:45
  • @P.E.Dant That is not correct. You may be interested in reading a book on this subject before commenting further, for example Geoffrey Leech's Meaning and the English Verb. – snailboat Sep 15 '16 at 22:45

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