“Which”和 “that” 都是引导限定性从句的关系词。限定性从句指的是作为主句中主语、宾语、表语、定语等补充说明成分的从句。它们的用法区别如下：
一、用法：通常来说，“that”更常用于引导限定性从句，而“which”则更适用于非限定性从句。例如：
1、You should only eat foods that are good for you. （用that引导限定性从句）
2、I've finished reading the book, which was really interesting. （用which引导非限定性从句）
二、逗号使用：“that”所引导的从句通常不需要使用逗号，而“which”所引导的非限定性从句需要使用逗号。例如：
1、The car that I want to buy is blue.（不需要逗号）
2、My new phone, which I bought last week, is really fast.（需要逗号）
三、替代：“that”可以替代特定的人或物，而“which”则不能。例如：
1、The car that I want to buy is blue. (不能用which代替，因为there may be many cars and "that”helps to specify which one)
2、I found a phone on the street which was really old.（可以用that代替）
四、关系代词的作用不同："Which”通常引导非限定性从句，并且通常在从句中充当主语或宾语；而“that”通常在限定性从句中充当主语或宾语。
1、This is the book that I was telling you about. （在限定性从句中充当宾语）
2、These are the flowers which I bought yesterday. （在非限定性从句中充当主语）
五、语气和口语用法：“that”比“which”更加常见，“that”可以用于书面语和口语。而“which”更通常用于书面语，在口语中不太常见。
which和that使用注意事项
一、在限定性从句中必须使用"that"、而不能使用"which"，因为这样可以明确指出是哪一个主语或宾语。例如：
1、I like the book that I borrowed from the library.
2、The car that we bought last week is really fast.
二、只有在非限定性从句中，才能使用"which"，而且需要用逗号与句子主体分开。例如：
1、My new watch, which is a gift from my friend, is really nice.
2、I had lunch with my friend, which was a really enjoyable experience.
三、如果从句中有介词，那么介词后面必须使用"which"，不能使用"that"。例如：
1、The room in which I stayed was clean and comfortable.
2、The book from which I learned a lot is now out of print.
四、如果从句中的关系代词作为宾语，可以使用"that"或"which"）与"who"作为主语。例如：
1、The people who/that live next door are really nice.
2、The apple which/that I had for breakfast was juicy and delicious.
五、如果从句中有多个名词，可以使用"which"，但不能使用"that"。例如：
1、The book, which is a bestseller, was written by a famous author.
2、The car, which has a leather interior, is more expensive than I thought.