类型一： if + 形容词 adj.
省略的主语和be，要注意主语和形容词对应。
- if true, this essay may change theory what we believed for a long time.
- Make a sign telling me if ready.
某些固定搭配：
- If necessary, ring me on my cellphone.
- If possible, let me know beforehand.
类型二：if + 过去分词 past participle
和上一条基本一样，这里就是把形容词的位置换成了过去分词，在高考英语中经常考主谓一致的时候会在这里出考题，请注意主谓一致。
- I will say if asked.
- Small trifles can escalate to a violence event if not paid good attention to.
- This fact proves that if put in the same position, women are able to earn as much as men do.
类型三：if + 不定代词 indefinite pronoun
这类省略比较依靠上下文，理解要靠语感。
- There are few people nowadays, if any, who remenber him.
- This seems to have little, if anything, to do with him.
类型四： if + 介词短语 prepositional phrase
有些固定搭配，理解主要依靠语境。
- If in doubt, ask your doctor, he can give you further information.
- Their policies have changed little, if at all, since last election.
- If by any chance you can't manage dinner tonight, perhaps we can at least have a drink together.
类型五：if ever
常和seldom连用，表示极少，万一，ever 后面可以接其他类型的成分（if ever 过去分词/形容词）
- He seldom sings if ever.
- I seldom do housework if ever.
- If ever you are in America, do give me a ring.
- The island is seldom if ever visited by ships.
类型六：if not/so
如果不/是
类型七：if need be
如果需要=if it is necessary
