ket核心语法：could you please open the door

1. 过去进行时

谈论过去某个时候正在发生的事情

I was playing football at 3:30 yesterday.

谈论过去同时发生的两件事

He was doing homework while his sister was watching TV.

过去进行时和一般过去时出现在同一个句子中：

Iwas doing homework when mysister cameto talk to me.

Someone knocked at the doorwhilemy mother was cooking.

表达否定句、疑问句：

He was not/wasn't working.

They were not/weren't speaking to each other.

Was he working?Yes, he was/No, he wasn't.

Were they speaking to each other? Yes, they were/No, they weren't.

2. can/can't/could/couldn't

有时，could可以作为can过去式使用

can/can't表示能力，用于当前时间

I canhelp you.((我可以帮助你)

could/couldn't过去时用于表示能力。

can/can't用于当前时间的表示概率

This can't be true.(这不可能是真的)

could表示概率，用于过去时，或用于现在时，但语气没有can强烈

He couldn't go therewithout me.(他不能不带我一起去)

can/could它也可以用于今天的请求。不同的是，它可以用于今天的请求could比can语气更加委婉礼貌

Can you help me please?

Could you please open the door?