could是情态动词。
aux. 能够， 可以可能差点就，本来有可能很想
v. 能（can 的过去式）
I had to pay a 500 deposit to the landlord before I could move into the house.
我得付给房东500英镑押金才能搬进房里去。
He had no money， nor did he know anyone he could borrow from， either.
他没有钱，也不知道能够向谁借钱。
There were about 500 people there， as near as I could judge.
那里有500人，依我看有这麽多。
So hard did he study that he couldeasily pass the examination.他那么努力，因而能够轻易地通过那次考试。
