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I am confused about this:

Can I use Must and Have to together like:

You must have to do it.

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    Just a note: must have to do... is possible. It's a matter of 'emphasizing' it further. – Maulik V Oct 16 '15 at 9:49
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    @MaulikV No, that's not correct. Your comment is a good example of why we should try not to answer questions in comments. A longer response won't fit in a comment, but I wrote a little bit about it in chat: chat.stackexchange.com/rooms/22937/conversation/must-have-to – snailcar Oct 16 '15 at 10:41
  • You mean in any context, 'must have to do' is incorrect? @snailboat – Maulik V Oct 16 '15 at 11:05
  • @MaulikV See below :) – Araucaria Oct 16 '15 at 13:32
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    @ChiragThakar You might want to wait a couple of days before accepting an answer, btw. You might get a much better one, but people are less likely to write another answer for you if you've already accepted one! :) – Araucaria Oct 16 '15 at 16:53
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Modal auxiliaries can have more than one sense. The verb must for example can be used to talk about obligations - what people think it is a good idea or bad idea to do. (Some people call this type of meaning deontic modality) Here's an example:

  • You must be in class by 9 am.

Here someone is saying that you have an obligation to be in class by 9 am. Probably that person has some authority, or they are using someone else's authority to tell you what you must do.

However, must can also be used to talk about whether we know that something is true or not. (Some writers call this type of meaning epistemic modality) We often use modals like this to talk about deductions - things that we know because of evidence:

  • This must be a 4, because that number's a 6.
  • Bob should be there by now, he left home at 2 pm.

We use have to to talk about necessity. In particular we can use it to express obligations:

  • You have to be in class by 9 am.

If we are using must to talk bout obligations, then we can't use it with have to. In other words, we cannot use must to make have to more emphatic. The following sentence cannot mean you really, really have to be in class by 9am:

  • *You must have to be in class by 9 am. (ungrammatical with this meaning)

However, if we are using must to talk about a deduction (if we are using it to express epistemic modality), then we can use it in a sentence with have to. The next sentence is grammatical if it means something like I logically deduce that it's necessary be in class by 9 am.

  • You must have to be in class by 9 am.

Here is the type of context that you might hear that sentence in:

A: The timetable doesn't say when I need to get there.

B: No, but it says that the break is at 10. The lessons are one hour long, so I'm guessing you must have to be in class by 9am.

  • What about taking the sentence like this: You must [have to] do it and You [must have] to do it? As I stated in other comment, I think assuming comes when we use must + have + past participle (compare She must have gone is assuming; She must have to go - almost mandatory). What am I missing? – Maulik V Oct 17 '15 at 4:11
  • @MaulikV We only interpret must in must have to as epistemic. Deontic must involves some kind of ergative illocutionary force. But the problem is that we cannot choose to have to do something, if we have to do it, it is just necessary, there is no choice involved. For example, I cannot tell you to have to breathe. It doesn't make any sense. – Araucaria Oct 17 '15 at 16:01
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Must or have to followed by an infinitive are used to express obligation. Hence, it doesn't make sense to use them together.

See here for a detailed explanation.

From the site:

We often use must for more personal opinions about what it is necessary to do, and have to for what somebody in authority has said it is necessary to do.

I must remember to get a present for Daisy. (my opinion)

You have to look after their hair regularly. (dog experts say so)

Do you have to wear a tie for school? (asking about school rules)

See here for usage of must and have to separately.

One more reference site.

EDIT:

As stated in this thread, it's possible but amounts to semantic overkill.

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    Why is this downvoted? Please state the reason. – Mamta D Oct 16 '15 at 9:03
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    +1 for the right answer. I am at a loss why it's downvoted. – Khan Oct 16 '15 at 9:21
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    Thanks. It would help if people mention the reason why they downvote so that it can motivate us to do better. – Mamta D Oct 16 '15 at 9:22
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    I didn't downvote, but I can't upvote the answer either. From this book: "We can use must have to to conclude something based on what we know about a present situation and must have had to to conclude something about a past situation". (I'm not sure whether the downvoter downvoted your answer because of the same reason, though.) – Damkerng T. Oct 16 '15 at 10:56
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I would point out that a common usage:

you must have to do _ to achieve _

For example:

You must have to reboot the computer to get the internet connection to work. Nothing else I tried seem to fix the problem.

The must here means you are obliged to. You have no choice. The have to x is stating the action that you are obliged to take. The sentence works without the must as:

You have to reboot the computer to get the internet connection to work

In the example the problem with the internet connection may have been a temporary problem at the internet service provider so rebooting may not have actually solved the problem. So even someone very knowledgeable about computers would choose to soften the strength of their statement with a qualification as they are not sure whether rebooting actually fixed things.

The must qualification softens the statement to be less authoritative. A longer way of saying the same thing would be:

In my judgement of the observed facts you have to reboot the the computer to get the internet connection to work. Nothing else I tried seem to fix the problem.

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