if and only if

used to introduce a condition which is necessary as well as sufficient

當且僅當

witches are real if and only if there are criteria for identifying witches.

當且僅當有標準識別女巫，女巫才是真實存在的。

if and when

at a future time (should it arise)

如果，萬一

if and when the film gets the green light, be sure you've read the book first.

萬一電影得到許可，千萬保證你已經讀過那本書了。

if anything

used to suggest tentatively that something may be the case (often the opposite of something previously implied)

如果是這樣的話（之前多已提及相反的情況）

I haven't made much of this—if anything, I've played it down.

對此我并沒有加以重視——如果是這樣的話，那我是看低它了。

if I were you

used to accompany a piece of advice

（表示建議）換作是我

I would go to see him if I were you.

換作是我，我會去見他。

if not

perhaps even (used to introduce a more extreme term than one first mentioned)

［用于提出更極端的說法］甚至可能

hundreds if not thousands of germs.

數以百計甚至可能數以千計的細菌。

if only

even if for no other reason than

即使只為了

Willy would have to tell George more, if only to stop him pestering.

即使只為了阻止喬治惹惱別人，威利也會告訴他更多的。

used to express a wish, especially regretfully

要 是…就好了

if only I had listened to you.

如果我聽你的話就好了。

if so

if that is the case

若是這樣