关系代词用来引导定语从句。它代表先行词，同时在从句中作一定的句子成分。关系代词有主格，宾格和属格之分，并有指人与指物之分。那么你知道吗?下面跟着我学习一下吧。
一、
1、引导
定语从句关系代词代表先行词，同时在从句中作一定的句子成分，例如：The girl whom I spoke to is my cousin. 跟我讲话的女孩是我表妹。该句中whom既代表先行词the girl，又在从句中作介词to的宾语。This is the pencil whose point is broken.这就是那个折了尖的铅笔。whose 指物，在限定性定语从句中作定语He came back for the book which he had forgotten.他重新回来都忘记了他的那本书。which指物，在限定性定语从句中作宾语，可以省略
2、先行词
which
例如：He said he saw me there, which was a lie.他说在那儿看到了我，纯属谎言。
3、其他用法
关系代词that在从句中作宾语或表语时可省略。
例如：I've forgotten much of the Latin I once knew.我过去懂拉丁语，2010年大都忘了。
He's changed. He's not the man he was.他变化很大，已不是过去的他了。
注释：关系代词在从句中作宾语时可省略，that在从句中作表语时也可省略。非限定性定语从句中，不能用that作关系代词。
二、语法注意
1.常用that 不用which的情况:
1先行词为不定代词all，little，few，much, anything, everything, nothing, something等时。
I will do all that I can to help you.
2先行词被序数词、形容词最高阶或the only, the very等所修饰或其本身就是序数词﹑形容词最高阶时。
He was the only person that joined the army in his village that year.
3先行词既有人,又有物时
They talked about the people and the things thatthey remembered in the school.
4当主句是以who 或which开头的疑问句时, 定语从句用that 引导, 以避免重复。
Who is the boy that was here just now?
2. 若被修饰的名词充当从句介词宾语时
介词可提前于引导词之前,此时的引导词只能用whom 或which.