used with past participles to form PERFECT tenses

〔与过去分词连用构成动词的完成式〕:

Our guests have arrived.

我们的客人已到。

Has anyone phoned?

有人打过电话来吗？

We've been spending too much money.

我们已经花钱太多了。

I hadn't seen him for fifteen years.

我已经十五年没看见他了。

'I hope you've read the instructions.' 'Yes, of course I have.'

“我希望你已经看过使用说明了。”“当然看过了。”

You haven't done much, have you?

你没做多少，是吗？

sb had better / best do sth, used to say that someone should do something

某人最好（应该）做某事:

You'd better phone to say you'll be late.

你最好打电话去说一声你要晚到。

We'd better not tell Jim about our plans just yet.

我们最好先别把计划告诉吉姆。

had sb done sth, [formal] if someone had done something

如果某人做了某事:

Had we known about it earlier, we could have warned people of the danger.

假如早点知道这件事的话，我们就能警告人们有危险了。