used to introduce a condition which is necessary as well as sufficient
当且仅当
witches are real if and only if there are criteria for identifying witches.
当且仅当有标准识别女巫，女巫才是真实存在的。
at a future time (should it arise)
如果，万一
if and when the film gets the green light, be sure you've read the book first.
万一电影得到许可，千万保证你已经读过那本书了。
used to suggest tentatively that something may be the case (often the opposite of something previously implied)
如果是这样的话（之前多已提及相反的情况）
I haven't made much of this—if anything, I've played it down.
对此我并没有加以重视——如果是这样的话，那我是看低它了。
used to accompany a piece of advice
（表示建议）换作是我
I would go to see him if I were you.
换作是我，我会去见他。
perhaps even (used to introduce a more extreme term than one first mentioned)
［用于提出更极端的说法］甚至可能
hundreds if not thousands of germs.
数以百计甚至可能数以千计的细菌。
even if for no other reason than
即使只为了
Willy would have to tell George more, if only to stop him pestering.
即使只为了阻止乔治惹恼别人，威利也会告诉他更多的。
used to express a wish, especially regretfully
要 是…就好了
if only I had listened to you.
如果我听你的话就好了。
if that is the case
若是这样