Grammar

[Neelam_kunwar]

I do not know the rule of must and may. Please tell me the rule of how to use may and must in sentences.

[rewboss]

"Must" is much stronger than "may".

You can use "must" to indicate an obligation: You must pass your driving test before you can drive a car on public roads.

You can use "may" to indicate permission: You may smoke in here.

You can also use "must" when you make a firm conclusion based on evidence: Jack says he was in library at the time, but Mabel says she saw him in the dining room; one of them must be lying.

"May" is then used to indicate that you're not sure if your conclusion is right, but it's possible: Jack may have murdered Lord Arlingworthy; he had the opportunity, but on the other hand he had no motive.

[Neelam_kunwar]

Thanks for this description.

[BobK]

Originally Posted by rewboss

"Must" is much stronger than "may".

You can use "must" to indicate an obligation: You must pass your driving test before you can drive a car on public roads.

You can use "may" to indicate permission: You may smoke in here.

You can also use "must" when you make a firm conclusion based on evidence: Jack says he was in library at the time, but Mabel says she saw him in the dining room; one of them must be lying.

"May" is then used to indicate that you're not sure if your conclusion is right, but it's possible: Jack may have murdered Lord Arlingworthy; he had the opportunity, but on the other hand he had no motive.

I agree; but I'd add that things change a bit in the negative:

As must indicates an obligation, must not indicates a prohibition.

But although may indicates permission, may not indicates rather more than just the withdrawal of permission: it can offer an option -

You may not want to go, but I'd advise it.

But it can also denote prohibition (no weaker than must not)

Children may not run in the corridor.

Things get worse when thinking about logical possibility/probability:

One of them must be lying, and it can't be Mabel.

Mustn't used to be acceptable here, so you may come across it in period writing, but it's not common (at least, not in BE) nowadays.

At least may and may not are in direct opposition to each other in this case:

It may be true and it may not be. (but watch the order: Maybe, maybe not.)

Oh dear - this is probably more information than you wanted.

b