It is...that通常不能强调哪些成分
请看一道题：
It was a teacher that she __________.
A. was B. seemed C. looked D. married
此题应选D。从句意上看似乎4个选项都行，其实这里只能填D。因为填A，B，C 中的任意一个选项，都会导致句中的 a teacher成为表语，然而按照强调句句法规则，It is...that这一强调句型是不能强调表语的。一般说来，以下成分不能用于强调句中的被强调部分：
1. 表语：
她很美。
正：She is beautiful.
误：It is beautiful she is.
他是医生。
正：He is a doctor.
误：It is a doctor that he is.
但是宾语补足语却可以用作被强调部分：
他们选他担任主席。
正：They elected him chairman.
正：It was chairman that they elected him.
2. since，as原因状语从句(但because从句却可以)：
他不能来是因为他病了。
正：It is because he is ill that he can’t come.
误：It is since[as] he’s ill that he can’t come.
3. although引导的让步状语从句：
虽然累了，但他还是去了。
正：He went although he was tired.
误：It was although he was tired that he went.