There are two tenses in English – past and present.
英语里有两种时态—过去时和现在时。

The past tense in English is used:
英语里过去时用于：

to talk about the past
谈论过去

to talk about hypotheses – things that are imagined rather than true.
谈论假设

for politeness.
基于礼貌

There are four past tense forms in English:

We use these forms:

to talk about the past:

He worked at McDonald’s. He had worked there since July.

He was working at McDonald’s. He had been working since July.

to refer to the present or future in conditions:

He could get a new job if he really tried.

If Jack was playing they would probably win.

and hypotheses:

It might be dangerous. Suppose they got lost.

I would always help someone who really needed help.

and wishes:

I wish it wasn’t so cold.

In conditions, hypotheses and wishes, if we want to talk about the past, we always use the past perfect:
在条件状语从句、虚拟语气以及表祝愿时，我们在谈论过去时，经常使用过去完成时。

I would have helped him if he had asked.

It was very dangerous, What if you had got lost?

I wish I hadn’t spent so much money last month.

We can use the past forms to talk about the present in a few polite expressions:
在一些礼貌的表达中，我们用过去式来谈论现在的事情。

Excuse me, I was wondering if this was the train for York.

I just hoped you would be able to help me.