語法書上應該有更為詳細的解釋。
這大概跟你說說兩種情況:
修飾物時只用which不用that的情況 :
Bamboo is hollow, which makes it light.竹子是空心的，這使得他很輕。
2. 當關系詞前有介詞時。例如：
This is the room in which Chairman Mao once lived.這就是毛主席曾經住過的地方。
3. 在一個句子中有兩個定語從句，其中一個定語從句的關係代詞用了that, 另一個宜用which.例如：
Let me show you the novel that I borrowed from the library which was newly open to us.我來給你看我從新開的圖書館裡借來的那本小說。
4. 當關系代詞後面有插入語時。例如：
Here is the English grammar which, as I have told you, will help improve your English.這就是我跟你說過的會有助於你學好英語的那本英語語法書。
5. 先行詞為that時。例如：
The clock is that which can tell us the time.鍾是報時的裝置。
只用that不用which的情況 :
1.當先行詞是all, little, few, much, something, everything, anything, nothing, none等不定代詞時。如：
Everything that happened then was like a nightmare.當時發生的一切就像是一場噩夢。
I will tell him all that you told me at the ball.我要把你在舞會上跟我說的話全都告訴他。
2.當先行詞被only, any, few, little, no, all, just, very（恰好的，表示強調）等詞修飾時。例如：
The only thing that we could do was to wait.我們能做的只是等待。
That"s the very thing that we can do.那正是我們能做的事。
3、當先行詞是序數詞、形容詞最高階或先行詞被序數詞或形容詞最高階修飾時。例如：
The first place that we visited was the Great Wall.我們參觀的第一個地方是長城。
This is the best novel that I have ever read.這是我看過的最好的小說。
4.被修飾詞為數詞時。例如：
Yesterday I caught two fish and put them in a basin of water. Now you can see the two that are still alive. 昨天我捉到了兩條魚，把它們放在一盆水裡。你可以看到那兩條魚還活著呢。
5.主句是There be結構，修飾其主語的定語從句宜用that作關係代詞修飾物。例句：
There"s still a room that is free.還有一個空房間。
6.先行詞中同時包括人和物時。例如：
We talked about the people and the villages that we remembered.我們談論了我們記得的人和村子。
He asked about the factories and workers that he had visited.他問起他去過的這幾家工廠和工人的情況。
7.當主句中有who, which時，而定語從句中也要用到who或which時，為了避免who…who, which…which等重疊，定語從句要用that引導。例如：
Who is the man that is standing by the door? 站在門邊的那個男人是誰？
Which of the two cows that you keep produces more milk? 你養的那兩頭奶牛中哪一個產奶多？
8.人或物在定語從句中作表語時，用that引導定語從句，而that通常也可以省略。例如：
He is no longer the man that he was.他不再是過去的他。
供參考。
語法書上應該有更為詳細的解釋。
這大概跟你說說兩種情況:
修飾物時只用which不用that的情況 :
引導非限制性定語從句時。例如：Bamboo is hollow, which makes it light.竹子是空心的，這使得他很輕。
2. 當關系詞前有介詞時。例如：
This is the room in which Chairman Mao once lived.這就是毛主席曾經住過的地方。
3. 在一個句子中有兩個定語從句，其中一個定語從句的關係代詞用了that, 另一個宜用which.例如：
Let me show you the novel that I borrowed from the library which was newly open to us.我來給你看我從新開的圖書館裡借來的那本小說。
4. 當關系代詞後面有插入語時。例如：
Here is the English grammar which, as I have told you, will help improve your English.這就是我跟你說過的會有助於你學好英語的那本英語語法書。
5. 先行詞為that時。例如：
The clock is that which can tell us the time.鍾是報時的裝置。
只用that不用which的情況 :
1.當先行詞是all, little, few, much, something, everything, anything, nothing, none等不定代詞時。如：
Everything that happened then was like a nightmare.當時發生的一切就像是一場噩夢。
I will tell him all that you told me at the ball.我要把你在舞會上跟我說的話全都告訴他。
2.當先行詞被only, any, few, little, no, all, just, very（恰好的，表示強調）等詞修飾時。例如：
The only thing that we could do was to wait.我們能做的只是等待。
That"s the very thing that we can do.那正是我們能做的事。
3、當先行詞是序數詞、形容詞最高階或先行詞被序數詞或形容詞最高階修飾時。例如：
The first place that we visited was the Great Wall.我們參觀的第一個地方是長城。
This is the best novel that I have ever read.這是我看過的最好的小說。
4.被修飾詞為數詞時。例如：
Yesterday I caught two fish and put them in a basin of water. Now you can see the two that are still alive. 昨天我捉到了兩條魚，把它們放在一盆水裡。你可以看到那兩條魚還活著呢。
5.主句是There be結構，修飾其主語的定語從句宜用that作關係代詞修飾物。例句：
There"s still a room that is free.還有一個空房間。
6.先行詞中同時包括人和物時。例如：
We talked about the people and the villages that we remembered.我們談論了我們記得的人和村子。
He asked about the factories and workers that he had visited.他問起他去過的這幾家工廠和工人的情況。
7.當主句中有who, which時，而定語從句中也要用到who或which時，為了避免who…who, which…which等重疊，定語從句要用that引導。例如：
Who is the man that is standing by the door? 站在門邊的那個男人是誰？
Which of the two cows that you keep produces more milk? 你養的那兩頭奶牛中哪一個產奶多？
8.人或物在定語從句中作表語時，用that引導定語從句，而that通常也可以省略。例如：
He is no longer the man that he was.他不再是過去的他。
供參考。