這要分成兩種子句來說明喔.
1.副詞子句: if做 "假如"解釋...若遇到未來式則以現在式代替未來式
ex: If Mary gets good grades on the test tomorrow, her mom will give her
this watch later.
2. 名詞子句: if做"是否"解釋..則遇到未來式還是使用未來式
ex: I don't know if you will come here tomorrow.
至於你說的關係子句的非限定用法..逗點後面不加that..
ex: I have one son, who lives in Taipei.
^^^不可加that
That is your cousin, isn't it?
此時的主詞是that,而that的代名詞是it,故還是要使用isn't it.
而還有另一種附加問句..
I don't know where they go, do I?
以主要子句的主詞和動詞為主..形成附加問句
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