现在完成时 the present perfect tense 

perfect 完成时的

I went to London in 2020.

I have been to London.

第一句是一般过去时（the past simple tense），在过去的某个时间点，已经完成某个事情、某个动作。一般过去时通常会有时间词，如 in 2020 或 last year 等等。

I went to London in 2020. 我2020年去了伦敦。

这句话的重点是，这件事发生在2020年，过去的这个时间点。

第二句是现在完成时（the present perfect），过去某个时间做的事情、动作，但它和现在是有联系的，它的影响、它的作用持续到现在。

I have been to London. 我去过伦敦。

这句话的重点是，去过伦敦这是我的一个经历，我曾经做过这个事，到现在还记得去伦敦的经历。它是什么时候发生的并不是重点。

现在完成时的形式

have/has + 动词的过去分词

肯定形式

I have worked. (I've worked.) 我工作过。

You have worked (You've worked.)

He / She / It has worked. (He's / She's / It's worked.)

We have worked. (We've worked.)

They have worked. (They've worked.)

否定形式

I have not worked. (I haven't worked.) 我没有工作过。

You have not worked. (You haven't worked.)

He / She / It has not worked. (He / She / It hasn't worked.)

We have not worked. (We haven't worked.)

They have not worked. (They haven't worked.)

疑问形式

Have I worked? 我工作过的吗？

Have you worked?

Has he / she / it worked?

Have we worked?

Have they worked?

简短回答

Yes I have. / No I haven't.

Yes you have. / No you haven't.

Yes he/she/it has. / No he/she/it hasn't.

Yes we have. / No we haven't.

Yes they have. / No they haven't.