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69．We are talking about if this plan should be carried out.
If 和whether都可以引导动词的宾语从句，如果从句做的是介词宾语，只能用whether引导。所以把if 改为whether。
70．I can not decide if to stay or not.
只有whether才能和不定式搭配使用。因此把if 改为whether。
71．My suggestion is we try for a second time
表语从句的引导词如为that，一般不省略。因此在we前加上that。
72．What will the professor say is not known yet.
名词性从句的语序都是陈述句语序。因此应改为：What the professor will say is not known yet.
