Could

Afirmativa: I could  play.

Negativa: I could not play.

Interrogativa: Could I play?

Expressa:

capacidade: She could swim years ago.

pedido polido: Could you leave, please?

Negativa: I cannot play. (can't)

Interrogativa: Can I play

Expressa:

capacidade: She can swin.

permissão/ possibilidade: You can leave now.

Afirmativa: I must play.

Negativa: I must not play. (mustn't)

Interrogativa: Must I play

Expressa:

obrigação: Everybody must come early.

dedução: You are pale. You must be sick.

conselho: You must see thet play.

proibição: You mustn't smoke here.

Must pode ser substituído por to have to:

He will have to come tomorrow.

I had to see her.