There are two tenses in English – past and present.
英語(yǔ)里有兩種時(shí)態(tài)—過(guò)去時(shí)和現(xiàn)在時(shí)。

The past tense in English is used:
英語(yǔ)里過(guò)去時(shí)用于：

to talk about the past
談?wù)撨^(guò)去

to talk about hypotheses – things that are imagined rather than true.
談?wù)摷僭O(shè)

for politeness.
基于禮貌

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:
在條件狀語(yǔ)從句、虛擬語(yǔ)氣以及表祝愿時(shí)，我們?cè)谡務(wù)撨^(guò)去時(shí)，經(jīng)常使用過(guò)去完成時(shí)。

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:
在一些禮貌的表達(dá)中，我們用過(guò)去式來(lái)談?wù)摤F(xiàn)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.