有一读者问： 在强调结构中，用which, when, where来代替that行吗？如：
(1)It is the verb to do which is often found in the active voice.
(2)It was in front of this altar of heaven where the emperor performed the three kneelings and nine prostrations before the altars of his ancestors.
(3)It was in 1949 when the People's Republic of China was established.
在英语强调结构“It is （was)+名（代）词+that”中，如果that之前的名词指物或抽象概念——正如其指人时可用who代替that一样——亦可用which代替that，所以上述句（1）并没有错。再如：
(4)It is the moon which we see there.
(5)It is his strong will which enabled him to work tirelessly.
至于例（2）与例（3）中的where与when则是英美人所常犯的错误，严格说来，都应改为that。
