There are two tenses in English – past and present.
英語里有兩種時態(tài)—過去時和現(xiàn)在時。

The past tense in English is used:
英語里過去時用于：

to talk about the past
談論過去

to talk about hypotheses – things that are imagined rather than true.
談論假設

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:
在條件狀語從句、虛擬語氣以及表祝愿時，我們在談論過去時，經常使用過去完成時。

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:
在一些禮貌的表達中，我們用過去式來談論現(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.