if 从句该用什么时态

请看看下面这道题：

I don’t know if he _________ or not tomorrow.

A. come B. comes C. will come D. is coming

【分析】此题容易误选B。认为 if 引导的是条件状语从句，从句谓语要用一般现在时表示将来意义。其实，此题应选C，句中if引导的不是条件状语从句(即if≠如果)，而是宾语从句(即if＝是否)，句意为“我不知道明天他是否会来。”请看以下类似试题：

(1) I don't know if she _________, but if she _________ I will let you know.

A. comes, comes B. will come, will come

C. comes, will come D. will come, comes

答案选 D，第一个 if 引导的是宾语从句，第二个 if 引导的是条件状语从句。

(2) “When _________ he come?” “I don’t know, but when he _________, I’ll tell you.”

A. does, comes B. will, will come

C. does, will come D. will, comes

答案选 D，第一个 when 疑问副词，用于引出一个特殊疑问句；第二个 when是从属连词，用于引导时间状语从句。

(3) “When he _________ is not known yet.” “But when he _________, he will be warmly welcomed.”

A. comes, comes B. will come, will come

C. comes, will come D. will come, comes

答案选 D，第一个 when 引导的是主语从句，第二个 when 引导的是时间状语从句。