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I am not completely sure about the use of 'must' 'can't' and 'should' in continuous aspect. It seems that different people have different opinions, especially regarding their use to refer to the future. Are the following sentences grammatically correct:

He must be earning a lot of money in a year's time.
He can't be earning a lot of money in a year's time.
He should be earning a lot of money in a year's time.
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I see no conflict in these statements.

In the first, "must" suggests the speaker knows something which leads to an almost-certain conclusion.

In the second "can't" suggests the speaker knows something which leads to an almost-certain conclusion (the opposite of "must") or that the speaker has evidence proving the statement.

In the third, "should" suggests the speaker knows, thinks, or perceives something which leads to a possible solution.

It is a matter of what concept is to be conveyed more than anything else.

The time frame itself does not affect the verb you are choosing. For example, these hold based on the explanation above:

He must be earning a lot of money in the next year.
He must be earning a lot of money in the next two years' time.

Note, that just replacing "year's time" won't work since the article "a", "the", etc., must also match the noun following. That is,

He must be earning a lot of money in a next year.
He must be earning a lot of money in a next two years' time.

Do not make sense.

  • You gave a clear explanation. It helped. But my confusion is related to the future reference. – Kaptan Singh Nov 11 '14 at 5:04
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    Can you clarify? Replacing the timeframe, does not change the use of the words you're asking about, "in a lifetime", "in a month", etc. would all lead to the same analysis. – Sylas Seabrook Nov 11 '14 at 5:05
  • I mean like, He must be earning a lot of money next month. If I replace 'in two year's time' with any other future time expression like 'next year', will your explanation still hold true? – Kaptan Singh Nov 11 '14 at 5:14
  • Close, but it's not a direct search-and-replace. I'll edit. – Sylas Seabrook Nov 11 '14 at 5:16
  • I know that we don't use the article, a, with 'next. If I edited, would it be correct? – Kaptan Singh Nov 11 '14 at 5:27
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Any time you want to use modals to talk about the future, you should use either simple present or present continuous tense.

  • He might go to bed.
  • He might be going to bed.

If you want to talk about the past, you should use either past perfect or past perfect continuous tense.

  • He might have gone to bed.
  • He might have been going to bed.

The actual meanings of the auxiliary verbs in each sentence don't change. Must still means must, can't still means can't, and should still means should.

Imagine these sentences.

  • He must be earning a lot of money in a year's time. He has no choice.
  • He can't be earning a lot of money in a year's time. It just isn't possible.
  • He should be earning a lot of money in a year's time, he has the boss's support and trust.

All three seem to make perfect sense to me, although simplifying them is probably more helpful to show how the verb tense works with the modal. Better to use a sentence like this to show how it works:

  • We must be swimming tomorrow. It is Tuesday, and we always swim on Tuesdays.
  • We can't be swimming tomorrow. We were just swimming yesterday.
  • We should be swimming tomorrow. If you want to meet us, stop by the pool.
  • He must be earning a lot of money in a year's time, if he expects to support his family. I think your interpretation of this sentence in not correct. I think it means that I am sure that he will be earning a lot of money in a year's time. – Kaptan Singh Nov 11 '14 at 6:54
  • Agreed. I changed the answer to reflect your input. – Andrew Lasher Nov 11 '14 at 7:04
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Let's look at these one at a time.

He must be earning a lot of money in a year's time. This is possible in the deontic sense of must. The speaker is imposing an obligation: It is necessary that he will be earning a lot of money in one year's time. The progressive form of the bare infinitive of EARN indicates that the earning of a lot of money will have begun in less than one year from now. While this situation may appear a slightly unusual, the grammar is fine. It perhaps seems more natural if we re-word it a little and provide some context:

A girl who is too young marry without her parent's permission tells her father she wants to marry. He says "You are both too young, and he earns far too little to support a wife. If you ask me in a year's time, I will consider giving my permission, but he must be earning more than £X a year".

We don't have to think up such contrived situations for The must + progressive aspect construction. Imagine a teacher who has to leave his students for a short time. She says "Behave yourselves while I am gone. You must all be working hard when I get back".

In the epistemic sense of logical necessity, must "does not normally permit future reference" (Leech, 2004,96, Meaning and the English verb.)

.

He can't be earning a lot of money in a year's time. That sentence is not possible when the three words in bold are used with their ability/logical necessity meanings, and I have been unable to find any examples of the construction being used for a future situation with the permission meaning.

He should be earning a lot of money in a year's time. This is possible with the deontic sense (weaker obligation than that conveyed by must). For examples, simply use should in the sentences I gave above for must. In the epistemic sense (weaker logical necessity than that conveyed by must), should, unlike must is possible with future reference: John bought some property to rent last year, It seems likely that the government is going to lift rent controls soon, so he should be earning a lot of money in a year's time.

  • I think they are all fine in the present time reference, even the one with 'can't', showing negative certainty. Am I right? – Kaptan Singh Nov 11 '14 at 10:49
  • @KaptanSingh - You are right. – tunny Nov 11 '14 at 10:56

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