could也能表示过去的一般的允许：
On Sundays we could(=were allowed to)stay up late.
星期天晚上我们能到较晚才睡。
当某一具体行为获准并付诸实施时，就必须用was/were allowed代替could：
I had a visa so I was allowed to cross the frontier.
我有签证，所以我被允许越过边界。
couldnt的用法比could要稍为广泛一些：
We couldnt bring our dog into the restaurant.
我们不能把狗带进饭馆里。
与此相反的意思应当用allow表示：
We were allowed to bring
允许我们把狗带进，
如欲使用完成时和被动语态，则必须用 allowed：
Since his accident he hasnt been allowed to drive.
自从他出了车祸以后就不允许他再开车了。
As a child he had been allowed to do exactly what he liked.
他小时候，容许他爱干什么就干什么。