Question about modal verbs

[sikwai]

I am a Secondary 3 student form Hong Kong,
and I would like to ask,
why the following happen.

Q1. Ask any question now as you may not talk during the test.

I want to ask, why it use may in the sentence, why not could?

Q2. It's impossible, they can't have finished it already?

I want to ask, why it use can't in the sentence, why not mustn't?

thanks for answering

[sikwai]

Can anyone help me??

[Vetty]

Hi

I am also a Newbie but I think I can answer for your first question.

Q1. Ask any question now as you may not talk during the test.

.......you could not talk during the test - means you are not able to talk during the test

This is not the correct meaning. A student can talk during his test but it is restricted to a certain extent.

For eg: the student can ask for water but he is not allowed to ask any doubts.

I request our senior members to correct me if I am wrong.

[David L.]

Ask any question now, as you may not talk during the test.

I'd like to ask, why would you/we use 'may' in the sentence - why not 'can'?

If we used 'can', it would suggest to the listener that as soon as the examination started, a magic spell was unleashed that robbed the students of the power of speech, so that they were totally unable to say anything.
'may' is used because what the supervisor is saying is that students do not have permission to speak during an examination period.

Q2. It's impossible, they can't have finished it already?

I'd like to ask, why is 'can't' used in the sentence, why not 'mustn't'?
Because the person speaking is meaning, it is a physical/mental impossibility for any person to have finished by now. Otherwise, using 'mustn't', he would be saying, you don't have permission to finish this quickly.