引导定语从句的关系代词有that,which,who（宾格whom），所有格whose）和关系副词 where when why 等，关系代词或关系副词在定语从句中充当一个成分，关系代词that，which,who,whom等在从句中分别作主语或宾语，whose在从句中作定语，而关系副词when,where,why等在从句中作状语。 如：
1.I will never forget the days when/in which we worked together.
2.I will never forget the days which/that we spent together.
解析：在句①中，表示时间的名词the days在从句中充当的是状语，所以用关系副词when来代指，引导定语从句修饰先行词the days；
同样，表示地点或原因的名词如果在从句中作状语，则用关系副词where 或why来代指；如果在从句中作动词的宾语，则用which或that来代替。 如：
1.This is the factory where/in which I worked.（作状语）
2.This is the factory that/which I visited years ago.（作宾语）
1.This was the first (when/what) I had serious trouble with my boss.
2.That is the reason (why) I did it.
3.This is the place (where) we met yesterday.
1.Mr. Jackson is the only foreigner that is present at the party.
2.He is one of the students who were praised by the teacher.
1.Along with the letter was his promise that he would visit me this coming Christmas.
2.Do you still remember the chicken farm that we visited three months ago.
解析：在句①中，that引导同位语从句解释说明promise的内容，that不充当任何句子成分，只起连接作用，而在句②中，that在其引导的定语从句中作动词visited的宾语，对先行词the chicken farm起修饰作用。
It's a pity that he don't come to give a speech.（形式主语）
We think it possible that you can finish the job today.（形式宾语）
2）谓语动词 appreciate, dislike, hate, like, love, make等接由if或when 引导的宾语从句时往往在从句前加上形式宾语it. 例如：
I would appreciate it if you could come to my birthday party.
3）动词hare, take, hide, punish, put等,后接由that 引导的病因从句式，往往在从句前加形式宾语it. 例如：
1.I take it that you will be leaving Shanghai soon.
2.we punished it that we had finished the project ahead of time.
4）短语动词answer for, count on depend on, insist on, see to等后接有that引导的宾语从句时，必须冠以形式宾语it. 例如：
① I'm counting on it that you will come.
② She'll see to it that he goes ahead.
1.If my lawyer had been here last Saturday, he would have prevented me from going.
2.If you had studied hard at school, you would be a college student now.