标准的强调句型是it is/was...that/...。其它连接词很少使用。有人认为被强调的成分如果是作状语的时间名词是也可以用when,例如：

It was yesterday that/when he left for Japan. 

但如果是介词短语作时间状语，则不能用when

It was in 1995 that he left for Japan. 不能用when.

有时候我们会见到which用于类似的结构中，其实这不是强调句型，而是which引导一个普通的定语从句。例如：

What is that on the desk?

It is a report which I am writing. (这不是强调句型。it指代问句中的that，which引导定语从句）

I have heard you are writing a book.

No, it is a report that I am writing. 强调句型。不能用which。

再比较下面二句：

I heard they sacked the secretary. No, it was the manager who was sacked.

(强调句型。）

Who was crying？ It was the manager who was sacked. (定语从句。）

偶尔见到的it is/was...where/which...通常都是定语从句结构，It为人称代词。why不可能用于强调句。

总之，强调句型用that最安全，可以用于强调所有成分。被强调的成分为人时，可以用who。最好不要再用其它词，以免出错。

有人认为，在美式英语中当被强调部分为物或抽象概念时，可用which代替that。总而言之，在没有把握的情况下最好都用that。

夸克《英语语法大全》（英文版P.1387）在将强调句型和定语从句比较时指出：

There are differences from relative clauses, however, in that the wh-forms

are rare in cleft sentences in comparison with that and zero although whose is allowed in cleft sentences (It's Uncle Bill whose address I lost), whom and

which are only marginally possible, and it is virtually impossible to use whom

or which preceded by a preposition. Thus:

It was the dog to which I gave the water.

can be read only as a sentence containing a postmodifying relative clause

(compare She was the woman to whom I gave the water), and not as a cleft

sentence.

即强调句型中与that和省略that相比较而言，除whose比较常用外，其它wh-词的使用较为少见。whom和which仅在极少的情况下使用。

正如夸克指出的，强调句型的标准用词是that（或省略that），如今不少人以为只要被强调的成分是人，就可以用who。这种认识会导致who的滥用。实际的情况是that远较who常用，who的使用是很少见的（夸克用rare表示）。作为学习英语的中国人，应该尽量使用that（或省略that），少用who. 其它wh-开头的词更不宜使用。因为我们自己很难判断那些少见的情况属于什么情况。