Must or other modals?

Question

Hello,Is it better to use have to, should and ought to  instead of must in these sentences? Would there be any difference  in meaning in either of these examples?This has to be John.This should be John.This ought to be John.Students have to pass an entrance examination to study at this school.Students should  pass an entrance examination to study at this school.Students ought to  pass an entrance examination to study at this school.You have to  take some medicine for that cough.You should take some medicine for that cough.You ought to take some medicine for that cough.The children should not play in the street.The children ought not play in the street.Thanks

MrBen · Accepted Answer

I imagine this first context is that someone is calling on the phone or knocking at the door. You are saying who you think the person is.

This has to be John. Maybe if everyone else has already arrived at the party and John is the only person left it could be. Or you are irritated that he hasn't arrived yet and so now you are saying this in an exasperated way.
This should be John. It's not such a common way to say it, but you are around 50-75% certain that it's him.
This ought to be John. This might be a little more common than "should be."
That must be John. This is what I'd say if I were quite certain the person on the phone/at the door was John.

Students have to pass an entrance examination to study at this school. This is mandatory and you are just stating a fact. You could use "must" here with a very similar meaning.
Students should pass an entrance examination to study at this school. This sounds like a complaint, perhaps there are a lot of bad students and this is your advice to fix this problem.
Students ought to pass an entrance examination to study at this school. Same as "should."

You have to take some medicine for that cough. This is very strong advice, or you are the person's mother or doctor. If you are the mother or doctor, you can also use "must"
You should take some medicine for that cough. This is just advice.
You ought to take some medicine for that cough. Same as "should."

The children should not play in the street. You are just giving your opinion.
The children ought not to play in the street. Again, this has the same meaning as "should" but we don't often use "ought to" in questions or negatives because the "to" is awkward.

Thanks You're welcome!

Nina_Nia · Answer

mrBenStudents have to pass an entrance examination to study at this school. This is mandatory and you are just stating a fact. You could use "must" here with a very similar meaning.

Thank you very much for your explanations.
I was confused by this type of examples. My textbook says that when something is a fact and not your opinion only 'have to' should be used and not 'must'.

MrBen · Answer

You're right! But if you were the principal of the school you could say both interchangeably.

Must or other modals?

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