What is the correct phrase for the blank(using "must"):

She walked for two hours. She ____________ tired.

I wrote "must be"; the correct answer was "must have been." I doubt that "must be" should be marked as wrong, though. Can someone tell me what the correct answer is? (And if both, what exactly is the difference in meaning?)

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    'She must have been tired' assumes that the walk took place in the past and the walker was tired at the end of it. 'She must be tired' implies that she is tired now. It's not impossible to say 'She walked for two hours' when speaking of a walk that has recently finished, though as a British English speaker I would probably say 'She has been walking for two hours'. – Kate Bunting Sep 26 '17 at 8:17
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    There often seem to be people posting queries like this on ELU. Your answer is by no means ungrammatical. As Kate says, it can even be the logical choice on occasion. But the other sentence would be the one met with on 95+% of occasions in real life. Without previous verbal context, one would expect 'She has been walking for [the last] two hours. She must be tired.' for the 'just finished walking' (or 'still walking') case/s. – Edwin Ashworth Sep 26 '17 at 8:25
  • So you are saying that both answers can be correct depending on the occasion, am I right? But if the walk took place in the past and she was tired at the end of it, why is it not 'She had walked for two hours. She must have been tired.'? – lolol Sep 26 '17 at 9:04
  • There is nothing wrong with using the past perfect here as you suggest, but the past simple is often used even in 'previous to then' cases. After all, what do you do when you have a sequence of three events in the past, devise a 'plu-pluperfect' for the first one? – Edwin Ashworth Sep 26 '17 at 9:53

Normally the difference is made clear by context. Absent additional context, either could be correct; it all depends on her current state of vigor, and ultimately what one intends to say. However, the most common case would arguably be that both sentences refer to the same point in time. Consequently, since the first sentence is in past tense, the second sentence should also be in past tense.

As pointed out in the comments, both are linguistically correct, but they have different meaning in terms of the point in time at which the tiredness is/was current.

She walked for two hours. She must be tired.

This implies that the two-hour walk ended recently, by the first sentence being written in past tense and also by stating that she is likely to currently be tired. Since it is "walked" in past tense, the two hour walk has clearly ended; she may or may not have reached her destination, but she is not currently walking (that would have made the first sentence something like "she has been walking for two hours", in present tense, at which point having the second sentence in past tense would be confusing).

She walked for two hours. She must have been tired.

This implies that the two-hour walk has ended at an unspecified, earlier point in time, same as above. It also implies that at that time (at the time when the walk ended) she likely was tired (which by implication was the reason for her to stop walking).

Take a somewhat more extreme example, by specifying a point in time for the walk, and by making the time at which she was/is tired explicit.

She walked for two hours a week ago. She must be tired now.

The two sentences may both be correct, but they are pretty obviously unrelated (the fact that she walked for a few hours a week ago is unlikely to cause her to be tired now).

She walked for two hours a week ago. She must have been tired then.

This is clearly talking about her state of vigor at the time of her walk. It's not unreasonable to be tired after a two hour walk, so the second sentence follows reasonably naturally from the first.

  • Note that the 'implications' come from the verbs and not the grammar (cf. "She slept for two hours. She must have been tired.", which reverses the causal order). Also, there is little implication that her fatigue terminated the walk -- she may simply have reached her destination (and anyone like her would be fatigued after walking that long). – AmI Sep 26 '17 at 18:48

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