请看下面一道题:I don’t know if he _________ 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。,句意为”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 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。