",which"有兩種用法
1.先行詞是特定或已知的人事物
(Ex:I love my girlfriend ,who has long hair)只有一個留長髮的女朋友
(cf:I love my girlfriend who has long hair)幾個女朋友裡面有留長髮的那個
2.表示前面一整件事
My roomate never picks up after himself, which irritates me a lot.
我的室友從來不整理,這件事使我非常煩躁
My roomate never picks up after himself," which" irritates me a lot.
You use which to refer back to an idea or situation expressed when you want to give your opinion about it.
"which "at the beginning pf a relative clause when specifying the roomate you are talking about, when giving more information about himDefining and non-defining clause describe it.