一、 That可以做副词，代词，形容词和连词，但是做连词时是最常见的，也是最难鉴别的， that 用作连词，引导名词性从句,状语从句和强调句。

①引导宾语从句。及物动词后的引导词that可省略。

I didn’t expect (that) he could win the championship.

The teacher pointed out that Tom was not working hard enough.

② 引导主语从句。通常采用it作形式主语的句型。

That the earth goes around the sun is known to everyone.

(It is known to everyone that the earth goes around the sun.)

③引导表语从句。

The trouble is that we are short of money.

④引导同位语从句。

引导同位语从句的that和引导定语从句的that是不同的。前者只起语法作用，在从句中不作任何成分;而后者在定语从句中作主语、宾语或表语。举例说明：

The news that he resigned from office surprised us.

The idea that he holds is very common nowadays on campus.

2. that引导状语从句

①引导目的状语从句。

Bring it nearer that I may see it better.

②引导结果状语从句。

What have I done that he should be so angry with me?

③引导原因状语从句。

I am afraid that I will fail in the driving test.

④引导让步状语从句。意为“虽然、尽管”。

Difficult that/as the task was, they managed to accomplish it on time.

⑤引导条件状语从句。意为“假使、假设”。

Supposing that you were in my position, what would you do?

On condition that you were lost in the desert, you should ask for help as soon as possible.

二、While可以做名词和连词，做连词时又可分为从属连词和并列连词。

做从属连词可以引导时间状语从句，让步状语从句，条件状语从句，

(1)引导时间状语

We must strike while the iron is hot.趁热打铁。

(2)引起让步状语从句，意思是“虽然…但是….”、“尽管…但是…”(多放于句首)

While he loves his students，he is very strict with them..

虽然他爱他的学生，可是他对他们很严格。

(3)引起条件状语，意思是“只要”

While there is life there is hope.

只要生命存在，就有希望。

While作并列连词用，意思为“而，然而”，表对比。而but表转折。

Honey is sweet，but the bee stings.

蜂蜜很甜，但蜜蜂有刺蜇人。

三、which可以做疑问代词和关系代词。

1. 用作疑问代词时，意思是“哪一个,哪一些”

Which is my seat? 哪个座位是我的?

2. 用做关系代词时，意思是“那一个,那一些”

He also had a gun with which to defend himself. 他还有一把自卫用的枪。

四、where可以做副词，连词，和代词。

当副词时，可分为疑问代词和关系代词：

1. (用作疑问副词)在哪里;往哪里;从哪里;在哪一点上

The driver asked me where I wanted to go.

司机问我想去哪里。

Where shall we go?

我们去哪里?

2. (用作关系副词,引导关系从句)在那里;往那里

That’s where we differ.

那是我们的分歧所在。

This is the city where I was born.

这是我出生的城市。

当连词引导地点状语从句，意思是“在...处;到...的地方;...的地方”

I’ll meet him where I first met you.

我将在初次见你的地方与他会面。

当代词，意思是“哪里,何处”

Where does he come from?

他是哪里人?