三、作强调词，构成强调结构用以帮助改变句子结构，使句子的某一成分受到强调。“It is (was) + 所强调的成分+ that (who) + 其它成分。”在这个句型中，it本身没有词义。在使用强调句型时需注意以下几点：
1. 请注意强调句型的特殊疑问句例When was it that he changed his mind to take part in the activity？
2. 在强调原因状语从句时，只能强调由because所引导的从句例It was because he was ill that he didn't come to school yesterday.
3. 在强调not … until结构时必须把not与until一起放到被强调的位置上例It was not until she took off her dark glasses that I realized she was my brother.
4. 注意强调句型与定语从句的区别例It was at 7 o'clock that he came here yesterday.( 强调句型)It was 7 o'clock when he came here yesterday.(定语从句)四、It用作形式主语It替代作主语的从句、动词不定式、动名词，而把真正作主语的从句、动词不定式、动名词置于句尾。
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