Czasownika must, który oznaczaja "musieć", używa się:

1. do wyrażenia konieczności i obligacji w teraźniejszości:

• You must complete this assignement on time. (Musisz ukończyć to zadanie na czas).

• She must study for the exam. (Ona musi uczyc się do egzaminu).

2. do wyrażenia stanowczych rad i zaproszeń:

• You must come and vist us. (Musisz odwiedzić nas.)

• He must see the Tower when he is in London. (Musi odwiedzić Tower kiedy jest w Londynie)

3. do wyrażenia prawdopodobieństwa w teraźniejszości:

• She must be at home at this time of the day. (O tej może powinna [musi] być w domu)

• They must be crazy. (Oni muszą byc stuknięci).

MUSTN'T, czyli MUST NOT

Mustn't nie znaczy "nie musieć" ale "nie wolno". Czasnownik MUSN'T = BE NOT ALLOWED TO.

• She mustn't interupt other students. (Nie wolno jej przerywać innym studentom.)

• You musn't smoke here! (Nie wolno tu palić).

Can I ask you a question?

May I have a piece of cake?

Could I get you to turn off the lights

Some other common phrases are

Do you mind if I open the window?

Would you mind if I asked you something?

Is it okay if I sit here?

Would it be all right if I borrowed your lawn mower?

Giving Permission

Sure.

Go ahead.

No problem

To ask permission                       Possible responses

Is it OK if . . .                                  I really wish you wouldn't.

          Do you mind if . . .                          No, I don't mind. Go ahead  

          May I . . .                                       Sure, no problem.

          Would it be a problem if . . .            No problem at all.

          Would it be OK if . . .                       No, please don't

                                                               I would prefer that you didn't.

"Can I do this, in this way?" - is about ability of the person asking

"Can you show me the way to the bus stop?" - is about ability (as in knowledge & ability of giving directions) of person being asked

"Can I borrow your pen?" - is a question of ownership.

Therefore the correct form would be:

"May I borrow your pen?"

"Can we have a party here?", the teenager asked his parents. - is a question of authority.

Therefore the correct form would be:

"May we have a party here?"