СПРОСИТЬ РАЗРЕШЕНИЕ
can, could (ИЛИ may – звучит несколько формально)
Can I use your pen?
Could we borrow your ladder, please? ~ Well, I’m using it at the moment (сould звучит вежливее, чем can)
May I see the letter? ~ Certainly.

РАЗРЕШИТЬ
can, may (NOT could)
You can wait in my office if you like.
Could I borrow your calculator? ~ Of course you can.
You may telephone from here (a written notice)

НЕ РАЗРЕШИТЬ
can’t, may not (NOT couldn’t)
Could we picnic here? ~ I’m sorry. I’m afraid you can’t.
Members may not bring more than two guests into the club.

ГОВОРИТЬ О РАЗРЕШЕНИЯХ
– которые устанавливает кто-то:
can, could
Present Simple:    Each passenger can take one bag onto the plane (Каждый пассажир может взять…)
Past Simple:         In the 1920s, you could drive without taking a test (В 20-е годы можно было водить машину…)
be allowed to
Present Simple:    Passengers are allowed to take one bag onto the plane (Пассажирам разрешается…)

Past Simple:         We weren’t allowed to look round the factory yesterday (Вчера нам не разрешили…)
Present Perfect:    They have been allowed to keep the Roman coins they found in their garden.
Future Simple:      Will I be allowed to record the interview on tape? (Мне можно будет записать интервью…)
Infinitive:  She is unlikely to be allowed to travel on that airline again (Врядли ей снова разрешат…)

– об ОБЩИХ разрешениях в прошлом
could ИЛИ was / were allowed to
Last century, women couldn’t / were not allowed to vote (… женщинам не разрешалось голосовать)
In the 1950s British children could / were allowed to leave school at the age of fourteen.
I could always stay / I was always allowed to stay up late as a child (мне всегда разрешалось)
Anyone could / was allowed to fish in the lake when the council owned it.

– о КОНКРЕТНЫХ разрешениях в прошлом
только was / were allowed to
Although he didn’t have a ticket, Ken was allowed to come in (NOT …could come in Хотя у него не было билета, ему разрешили…)
I was allowed to leave work early yesterday (Вчера мне разрешили…)
We were allowed to go into the control room when we looked around the power station (Когда мы осматривали… нам разрешили…)
That day he was not allowed to see his wife.