It was the first time that I spoke in public.这是我第一次在公共场合发言.

It was the first time that I spoke in public.这是我第一次在公共场合发言.
这个that是什么性质?
定语从句哪一内容

原句可变为 I spoke in public for the first time.而您提出的句子则是对这句话中the first time 的强调
这句话是明显的强调句句型 It is .that.
固定句型 记住即可
总结强调句型的使用：
一、it强调句型的构成：
It+be+被强调部分+that(强调部分指人做主语时时用who,指人做宾语时用whom)+句子的其他部分.
例如：
Mary met an old beggar in the street yesterday.
It was Mary who/that met an old beggar in the street yesterday.(强调主语)
It was an old beggar whom/that Mary met in the street yesterday.(强调宾语)
It was yesterday that Mary met an old beggar in the street.(强调时间状语)
It was in the street that Mary met an old beggar yesterday.(强调地点状语)
注意：it强调句型不能强调句子的谓语.
二、not…until结构强调句型的构成：
It+be+not+until部分+that+句子的其他部分.
例如：
He didn't go to bed until his father came back..
变为强调句型为：
It was not until his father came back that he went to bed.注意原句中的didn't go部分中的not提前后,剩下did go变为went.
本文开头提到的两个句子都是not..until结构用于强调句型.那么把这两句话变回正常的语序则为：
1)…but he didn't make his most important discovery until after the war.
2)The importance of Fleming's discovery was not fully recognized until World War II.
一、强调句型的用法
在英语中,我们常用It is／was… who/that结构来突出强调句子的某一成分(一般是句子中主语,宾语或状语).在这个句型中,it没有词汇意义,只是引出被强调的成分.如果被强调的成分是表示人的词,用who或that来连接都可以.如果是其它成分,则一律用that来连接.
E.g.It is my mother who／that cooks every day.
E.g.It was yesterday that Tom passed in the maths exam．
二、使用本句型的几个注意事项 1．被强调成分是主语,who／that之后的谓语动词应该在人称和数上与原句中的主语保持一致.
e.g.It is I who am right．
e.g.It is he who is wrong．
e.g.It is the students who are lovely．
2．即使被强调成分是句子中的时间状语,地点状语,原因状语等,也不能用when,where,because,要用 that.
E.g.It was after the war was over that Einstein was able to come back to his research work again .
E.g.It was in front of the bank that the old lady was robbed．
E.g.It was because of the heavy rain that he came late．
3．如果被强调的成分有同位语,同位语也应提前.
It was from him,his chemistry teacher,that Paul learned to watch carefully in class．
4．被强调成分是作主语的代词,用主格,是作宾语的代词,用宾格
It was he that helped me yesterday.It was me that he helped yesterday.
5．被强调部分既包括人又包括物,用that不用who
lt was the things and people that they remembered that they were talking about．
三、区分定语从句和强调句型某些定语从句和强调句型形式差不多,容易混淆.如果去掉it is／ was ...that句子仍然通顺成立,则为强调句型,不成立不通顺,则为定语从句.
E.g.It was three years ago that he went to American for a further study·去掉It was that句子为Three years ago he went to America for a further study．句子通顺,意思完整,那么,这就是一个强调句型