【1】that 作从属连词引导名词性从句：无意思，不作成分，只起连接句子作用，有时候可以省略，有时候不能省略：

A.引导主语从句。例如：It's surprising that she is [should be] so angry. 她那么生气，令人惊讶。/It is natural that she should cry. 她大哭是人之常情。/That he is a doctor is true.= It is true that he is a doctor.千真万确，他是医生。【注意】引导主语从句的that被省略的情况很少，特别是在句首时绝对不能省略。

B.引导宾语从句，作及物动词、某些形容词、某些介词的宾语。例如：I know that he doesn't love me. 我知道他不爱我。/The teacher taught us that we should always do our best. 老师教我们要时刻尽己所能。/ I'm sure that he is clever.我确信他是聪明的。 / I enjoyed the concert except that the hall was cold.我很欣赏那场音乐会，只是会场冷了些。【注意】（1）这个that在口语中常常可以省略。但是 that 引导两个或者多个名词性从句时，只能省略第一个that,以免意思变得模糊不清。例如：He said (that) there were lots of things to do and that he wouldn't go with us. 他说他有很多事要做，所以不和我们一起去。(2) 如果宾语从句作要求接宾语补足语的动词的宾语，通常在动词后面用it作形式宾语。例如： I found it impossible that I silenced her.我发觉自己无法使她安静下来。

C.引导表语从句。例如：The fact is that he doesn't even take the exam .事实是他连考试都没有参加。

D.引导同位语从句。The rumor that they would get married spread at once .他们要结婚的谣言马上传了开来。

【2】that 作关系代词，引导定语从句并且在从句中充当句子成分，可以作主语，宾语，表语等。

A.作主语，一般不能省略。例如：The building that stands over there is a hospital.坐落在那边的建筑物是医院。

B.作宾语，通常可以省略。例如：These are the pictures that I took in China.这些是我在中国拍的照片。

C.作表语。例如：She is not the girl that she was ten years ago. 她已经不是十年前的那个女孩了。

【3】that 作从属连词引导状语从句：

A.(表示目的) 以便......，为使...... （通常使用so that ...或 in order that ...形式）：I took a taxi so that I would be in time for the appointment.我坐出租车去，以便赶上约会。Come closer in order that you may see the screen better.靠近一些，以便你能更清楚地看到银幕。

B.(以so...that...或such...that...形式，表示结果、程度) 很 ...... 所以 ......；那么......以致...... ：I was so tired that I couldn't keep awake. 我很疲倦，所以不能保持清醒。She was such an excellent secretary that her boss tried to dissuade her from resigning.她是个非常优秀的秘书，所以她的上司设法劝她不要辞职。

C.(以so that...形式，表示结果) 所以 ......（在口语中 that 常可省略）: I slept well, so that I felt much better afterward.我睡了一个好觉，随后感觉舒服多了。

说明：在解答过程中，主要参考了外研社、建宏《英汉多功能词典》。