句中省略的是that,而不是who，从句子中可以看出强调的部分在句子中充当宾语。
同that引导的定语从句一样，当在从句中做宾语时，是可以省略的。
强调句型的构成是：It is (was) + 被强调部分+ that (who) + 句子的其他成分。被强调的部分放在 It is (was) 之后，其它部分置于that之后。被强调部分可以是主语，宾语，表语或状语。强调的主语如果是人，可以由who代替that。
1. 被强调的成分举例：
原句：Tom found my pen in the classroom yesterday.
强调主语：It was Tom who / that found my pen in the classroom yesterday.
强调宾语：It was my pen that Tom found in the classroom yesterday.
强调地点状语：It was in the classroom that Tom found my pen yesterday.
强调时间状语：It was yesterday that Tom found my pen in the classroom.
2. 强调句型的一般疑问式：
直接把is或was提到it之前即可。例如：
Was it Tom that found your pen in the classroom yesterday?
3. 强调句型的特殊疑问式：
特殊疑问词+ is (was) it + that +句子的其他成分。特殊疑问词即是被强调的成分。例如：
Who was it that found your pen in the classroom yesterday?
4. that (who) 有时可以省略：
这种强调句型中的that或who有时可以省略。例如：
It was my brother (that / whom) you saw the other day.
5. 强调原因状语从句要注意：
若从句由as或since引导，强调时则改为because，这是因为，because引导的原因从句表示的意义非常强烈，符合强调句的目的。例如：
As she got up late, she missed the first bus. 变为：
It was because she got up late that she missed the bus.
6. 强调句的否定转移：
有些否定句在变成强调句时，要把否定转移到被强调的词语之前。尤其是“not... until...”句式的强调要特别注意。例如：
He didn't realize his mistake until the teacher had told him. 变为：
It wasn't until the teacher had told him that he realized his mistake.
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