3．强调：
强调是一种修辞方式，如前面所说的"倒装"就是一种重要的强调手段。
此外，还可以借助助词进行强调，例如：
She did come yesterday; I saw her in the office.
（她昨天确实来了；我在办公室见到她的。）
本讲要讨论的主要是"it is ….that…"强调句, 即：将句子中除谓语动词以外的成分（主语、宾语、状语等）用"it is ….that…"加以连接，组成强调句，如果是人，也可以用"it is… who…", 例如：
John bought a toy train for his son yesterday.（昨天约翰给他儿子买了辆玩具火车。）
强调主语：It was John who （或that）bought a toy train for his son yesterday.
强调宾语：It was a toy train that John bought for his son yesterday.
强调宾语（或目的）：It was for his son that John bought a toy train yesterday.
强调时间状语：It was yesterday that John bought a toy train for his son .
学习强调句要注意以下两点：
不要与主语从句相混淆，例如：
It is well known that light travels in straight lines.
（众所周至，光以直线传播。—— 主语从句）
It was not until the early 18th century that the significance of the event was generally realized.
（直到18世纪初，这一事件的意义才为公众所认识。——强调句）
强调句与主语从句的简单区分是：强调句去掉"it is…that…",句子结构依然正确，意思不变，如上句："until the 18th century the significance of the event was not generally realized". 但是主语从句就不行，如上句："well know that light travels in straight line"就不通, 句中 "well know" 变成了无依着的成分。
不要陷入"考试陷阱"，例如：
It was not until 1943 ______ Penicillin was put into clinical use that many fatal diseases were brought under control.
A.that
B.when
C.before
D.after
看到这个题目，考生第一个反应是"强调句"，很快选择了A，但是再仔细读一下，句子后面已经有"that"，说明这个题目的考点不在"强调句"上，而是考"定语从句"的"关系副词"when, 故正确答案是B，A称为"干扰项".