1 如果现行词是物，并且在
从句中作宾语时，
关系代词可以用that / which 或者不用关系代词都可以，例如：
(1) The book that he bought for me was lost yesterday.
(2) The book which he bought for me was lost yesterday.
(3) The book he bought for me was lost yesterday.
以上三句话中现行词The book在从句中作宾语，有三种写法，都正确，而且意思一样。
2 如果现行词是人，并且在从句作宾语，关系代词可以用whom / who / that或者不用关系代词都可以，例如：
(1) The man who I was talking to just now is my English teacher.
(2)The man whom I was talking to just now is my English teacher.
(3) The man that I was talking to just now is my English teacher.
(4) The man I was talking to just now is my English teacher.
3 如果现行词既有人又有物，关系代词只能用that或者不用关系代词，例如：
(1) The places and people that I visited during my summer vacation are wonderful.
(2) The places and people I visited during my summer vacation are wonderful.