定语从句和强调句的区别如下：
强调句中的It没有实际意义，It be与that可同时被省略；而定语从句中的It是主语，It be与that不可同时省略。
强调句型中be的时态须跟后面句子的时态相一致；而定语从句中主句谓语动词be的时态须由主句的时间确定。
强调句将句子中的that不能省略，并且即使前面的名词是事物时，也不能将that换成which；而定语从句中的that作宾语时可被省略，并且当先行词是事物时可用which代替。
当it be后面的时间、地点名词作主语、宾语或表语时，引导词可用that / which；而作其他成分时，引导词须用when / which。
例句：
1、It is an English book (that / which) I bought yesterday.
它是我昨天买的书。
(本句是对What is that?问句的回答，that所引导的是定语从句，that可被省略。)
It was the English book that I bought yesterday.
昨天我所卖的就是这本书。
(本句相当于对I bought the English book yesterday中an English book进行强调。)
2、It was a room where we used to have meetings.
它是我们过去常在开会的房子。
(where 所引导的从句对前面的room进行说明，它是定语从句。)
It was in the room that we used to have meetings.
过去我们开会就在这间房子里。
(in the room是被强调的部分，本句是强调句型。)
3、It is a day when the people celebrate their victory.
这是人们庆祝他们的胜利的日子。
(when所引导的从句对a day进行说明，是定语从句。)
It was on that day that people celebrated their victory.
就在那一天，人们庆祝了他们的胜利。
(强调on October 1, 1949。)