【Key structures】Can and Be able to

情态助动词 can/could 用于表示请求别人允许或答复时的情况:

Can/Could I borrow you pen? can 有时可以表示可能性：

I can have lunch with your tomorrow if you like.

在表示天生的或学到的能力时，can/could 可用于现在时和过去时，can/could 与 be able to 通常可以互换，

表示将来的“能力”时，则用 will be able to： I could drive a car by the time I was sixteen.

I tried again and found I could/was able to swim. Can Jane swim yet?

No, but she’ll be able to swim in a few months’ time.

在谈到说话时正在发生的事时，一般不用 be able to： Look! I can stand on my head.

在表示成功地完成某一具体动作时，通常不用 could，而用 be able to；如果表示某一动作没有取得成功，

则可用 couldn’t：

It’s pity he couldn’t visit Mary.

Oh, didn’t you know? He was able to visit her after all.

He was able to leave Europe before the war began.                                                                                         他在战争开始之前得以离开欧洲。

He was able to go to London yesterday and he enjoyed himself very much.

在问及过去某一具体活动时可用 could，但回答是肯定的时候则不能： Could he borrow a car?

Yes, he was able to borrow a car after all./No, he couldn’t.

【Special  Difficulties】

含有介词 at 的词组

at 与许多词可以构成固定词组，其中表示时间的有：at first（开始时，最初），at once（马上，立刻）， at present（目前，现在），at last（最后，终于），at times（有时），at the moment（现在）；表示地点的有： at home（在家），at school（在学校）；表示程度的有：at least（至少），at any rate（不管怎样）；其它词组有 at heart（内心里，实际上），at a loss（困惑不解，不知怎么办）等。

At any rate, I’m determined to stay here at present. At least, I’m not mad yet.

People have more money now, yet the temptationto to steal is greater than ever before. Why? I’m at a loss. Though he tried to made conversation with the lady, he wasn’t interested in her at heart.