首先，定语从句还有另外一个名称——形容词从句，其目的是修饰名词即先导词，这是定语从句的特点，没有先导词，定语从句就失去存在的意义。二者缺一不可。
其次，强调句顾名思义就是要强调句中某个成份，通常是主语、宾语或状语。与定语从句容易混淆的是其中一种常见格式，It is(was) ... that(who, whom) ... ，其特点是强调那部份，就到 that 前面做 it is 的受词。如：
It is John who/that broke the window yesterday.
It is the window that John broke yesterday.
It is yesterday that John broke the window.
1. 强调句与主语从句的比较
强调句将句子中的it is / was…that同时省去，句子仍然成立；而主语从句将that与后面部分代替it，成立。如：
(1) It is Li Lei’s brother that you met in the street yesterday. 你昨天在街上所见到的就是李蕾的兄弟。
本句若将It is 及 that同时省去为：You met Li Lei’s brother in the street yesterday. 句子同样成立，因此前面的句子是强调句型。
(2) It is exciting that we have succeeded in sending up Shenzhou V. 我们成功地发射了神舟五号这件事情真令人兴奋。
本句若将It is 及 that同时省去为：We have succeeded exciting in sending up Shenzhou V. 显然句子错误，因此，前面句子不是强调句型，而是主语从句。
2. 强调句与定语从句的比较
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。如：
(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)