这里talk 是个不及物动词, 英语中表示谈论某事 就要用 talk about sth , 及物动词后可以直接跟宾语, 而不及物动词不能直接跟宾语,要跟宾语就得借助于介词.
This is a subject which/that we have talked about a lot.
This is a subject about which/that we have talked a lot.
以上句子是定语从句,subject 是先行词 在定语从句中作talk about 的宾语 , 所以 就得加上about 句子才是正确的. 英语中不可以说 talk sth .