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Please tell me if the following sentences are the same or not?

It is ten o’clock. They must have arrived now.

It is ten o’clock. They must have arrived by now.

Are they same? If not, then please tell me what the difference between them is.

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  • I can't see any real difference in meaning, but your second example is perhaps the 'usual' way of expressing one's concern about such things. Perhaps "now" is more common than "by now" in AmE.
    – BillJ
    Commented Apr 30, 2016 at 18:46
  • Must have arrived by now sounds better to my ears. Consider should as replacing must in certain cases
    – sbswim
    Commented May 1, 2016 at 13:49

2 Answers 2

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They mean different things about a possible event in a past interval that extends to the present time. The first is an inference from evidence. Suppose you're awaiting guests and you hear a commotion in the hallway outside your door. You say, "They must have arrived now", meaning that the noise indicates that your guests are here.

The second is more speculative and concerns a deadline. Suppose your friends are traveling by train, and their train is scheduled to arrive in the station at 1:00PM. When it's 1:15PM and you say, "They must have arrived by now", you mean that it's reasonable to conclude that your friends are at the station because the scheduled arrival time has passed. You don't actually know that, of course. The train might be late or your friends might have failed to make the train at its departure.

Note that neither of these usages expresses an aspect of compulsion as when your friends actually arrive at your apartment and you tell them "You must take your shoes off before you enter."

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  • In both of your two cases, you're stating a conclusion you draw from evidence. I don't see any difference, except in what evidence you use. And anyhow, your first case, where the evidence is commotion, is not relevant to the question, since it ignores part of the example, which tells us the evidence is the current time.
    – Greg Lee
    Commented Apr 30, 2016 at 17:57
  • @GregLee You don't see any difference between physical evidence (noise) and the abstraction of a schedule? The latter isn't evidence of what happened, just of what should happen. I'm not sure what to tell you. I wouldn't use the phrasing "They must have arrived now" when talking about a deadline. Would you? Sorry, but I'm not following your "anyhow". Obviously, the commotion is happening a the time of the statement, otherwise now would make no sense. Not to mention that current time isn't evidence of anything. Are you saying that the first statement is ungrammatical or nonsensical?
    – deadrat
    Commented Apr 30, 2016 at 19:10
  • You keep ignoring the first sentence of the example, "It is 10 o'clock." Given that context, the "now" and "by now" seem to me to have the same sense. Neither could plausibly refer to the time of some sudden commotion. Now is the current time -- 10 o'clock.
    – Greg Lee
    Commented Apr 30, 2016 at 20:41
  • @GregLee I'm not ignoring anything. I'm telling the OP when you would use now and when you would use by now. Is 10 o'clock a deadline? Then what does my answer say to use? What's so hard about this?
    – deadrat
    Commented Apr 30, 2016 at 22:36
  • It's not hard. I think you've been very clear. But that is not the same as being correct. Taking into account the context given in the question, it's my opinion that your clear answer is wrong. Is that clear?
    – Greg Lee
    Commented Apr 30, 2016 at 23:15
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"They must have arrived now" is not correct, and doesn't exactly mean anything. "They must have arrived" is the Conditional II Simple tense (see https://www.ego4u.com/en/cram-up/grammar/conditional-2-simple), which refers to action which could have taken place in the past -- or in this case, must have taken place.

So "now" doesn't fit in this sentence.

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