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In general, do you use "must" or "have to" in Time clause?

For Example: can I say, "After you must clean the floor, you can go out".

I knew that we are not allowed to use future tense (will, be going to) in time clause, but what about "must" or "have to"

One of my students said that?

I am an English assistant for a native English teacher!

  • I am an English assistant! – Tom Sep 28 '16 at 5:01
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You don't use must/have to in time clauses that start with a conjunction like before, after, when, until, as soon as, etc. You use them in the main clause. For examples:

You must go out after you have cleaned the floor.

You have to/must do your homework before you go to bed.

  • Do you have any referencing source to back your claim? – Tom Sep 28 '16 at 9:17
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    +1. @Tom: Modals are outside of finite time. You must eat food and drink water to survive. That sentence does not refer to an event. Temporal phrases like "after" and "before" make no sense when used in relation to that which is not an event happening in finite time. "After you must eat food and drink water to survive" is a meaningless utterance. After this coffee is from Ethiopia. makes no sense. – Tᴚoɯɐuo Sep 28 '16 at 9:47
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I think the student wanted to use the present perfect form of the verb clean

After you have cleaned the floor, you can go out

Otherwise

You must / have to clean the floor before going out.

After is a preposition, it precedes a noun or "that" which substitutes the noun

After dinner OR After that, you must / have to clean the floor.

  • But, in general, are we allowed to use must in Time clause? – Tom Sep 28 '16 at 5:13
  • @Tom the modal verb must can only be used in the present tense, its equivalent have to can be used with different tenses. My second example with "before" is a time clause. The last example, is also a time clause, but the word after should precede a noun or a pronoun. – Mari-Lou A Sep 28 '16 at 5:31
  • e-grammar.org/time-clauses – Mari-Lou A Sep 28 '16 at 5:37
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Since

must
have to

usually define an existing and necessary condition, you could phrase your sentence as

First, you must clean the floor, then you may go out.
First, you have to clean the floor, then you may go out.

Both sentences have the same meaning.

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