if的用法是什么,当解释为是否时用什么时态

if /ɪf/ CET4 TEM4
位于句首常读作ˈɪf.
1.
CONJ You use if in conditional sentences to introduce the circumstances in which an event or situation might happen,might be happening,or might have happened.(引导条件从句) 如果
例：
She gets very upset if I exclude her from anything.
如果有什么事我将她排除在外,她就会非常难过.
例：
You can go if you want.
如果你想走就走吧.
2.
CONJ You use if in indirect questions where the answer is either "yes" or "no." (用于间接是非问句中) 是否
例：
He asked if I had left with you,and I said no.
他问我是不是和你一起离开的,我说不是.
3.
CONJ You use if to suggest that something might be slightly different from what you are stating in the main part of the sentence,for example,that there might be slightly more or less of a particular quality.(表示与主句所述事实可能有少许不同) 即使
例：
Sometimes that standard is quite difficult,if not impossible,to achieve.
有时那个标准,即使并非不可能,也是很难达到的.