It is ···that···是强调句的句型，起到强调的作用，也可以省去。比如说：
1.It was last night that I saw the comet.是强调时间（last night）的。
原句就是I saw the comet last night.
如果强调看的东西（comet)的话就是1.It was the comet that I saw last night.
同理
3.It was at eleven that he came back.的原句就是
He came back at eleven.
而2.It was eleven when he came back.是when引导的时间状语从句。
这么说你明白了吗，如果不明白我在详细解释一下。
可以，就看你要表达什么意思，when的话就只是在说时间，剩下的是辅助信息，在口语中说是可以的，但是后面一般还要再加你要说明的信息 比如It was last night when my father came to see me that i went out.书面与一般用that
that的话是强调时间，但是剩下的也是重要信息。
It is ···that···是强调句的句型啊
1.Jim
is
often
late
for
work.
2.The
little
girl
is
afraid
of
seeing
the
dentist.(be
afraid
of
后面不能跟句子，只能跟名词短语，表示害怕什么。。。，
seeing
the
dentist是动名词短语。
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