如何区分主句与从句?像是一般的比较好区分,但比如no sooner……than的主句为什么在前面呢

1.状语从句,主句没有引导词,从句有一般为 when,if,though
I don't know when you will come
2.名词性从句（主语、宾语、表语、同位语）找到先行词或引导词.（有时候引导词会省）
where we will go haven't been decided.（主语从句）I don't know where I will go.（宾语从句）
The trouble is (that) he is ill（表语从句）
I have no idea when they will come .（同位语从句）
3.定语从句,找到先行词.
I know the man who wrote the book.