He is very busy. I dont know if he _____ or not tomorrow.
A. come B. comes
C. will come D. is coming
【陷阱】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。
【分析】其实，此题答案应选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)，句意为他很忙，我不知道明天他是否会来。请看以下类似试题：
(1) I dont 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 dont know, but when he _____, Ill 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。