it was that 引導的是表語從句；it was when引導的是賓語從句

強調句用it is...that還是It is...when的問題

例：

It was later when I began to realize he had many good qualities that I lacked, but we made up and became frieds again.

從句子意思和結構上看，本句屬於強調句型，強調了時間副詞later，應把when改為that。

It was later that I began to realize he had many good qualities that I lacked, but we made up and became frieds again.

make up為固定片語：“和解，言歸於好”。

例：

It was only in the last decade when both political and social changes took place that women were finally granted the freedom to love who they wanted to love.

it was that是強調句型,強調的是 only in the last decade when both political and social changes took place ,而這個被強調的部分在原句中做的是時間狀語從句的成分.
when在decade後是定語從句修飾decade,可以翻譯成在經濟和社會變革發生的最近的十年裡,.
整個句子可以去掉強調結構就是Only in the last decade when both political and social changes took place ,women were finally granted the freedom to love who they wanted to love .只有在經濟和社會變革發生的最近的十年裡,婦女才最終被賦予能夠愛她們所愛的人的自由

It was... that...是強調句，它就是個句套子，把它省略，句子結構，意思都是完整的，

It was yesterday that i met Mary.強調的是我遇見瑪麗是昨天，強調昨天

It was yesterday when i met Mary.這是個定語從句，yesterday是先行詞，在從句i met Mary中，它充當時間狀語，所以用關係副詞when

雖然中文意思差不多，但是語法上還是有區別的。