Question

It was October _when__ the People’s Republic of China was founded

It was October __when_ the People’s Republic of China was founded
It was in October__that_ the People’s Republic of China was founded.
为什么

Answer 1

第一个句子：It was October when ...这里的 when 引导定语从句，修饰 October，全句直译是：那是中华人民共和国成立的十月。（当然，此句需要意译才会更通顺）。
第二个句子：It was in October that ...是一个 it was...that 的强调句型，它的陈述句是 The People's Republic of China was founded in October. 改成强调句型后不能漏掉陈述句中的任何一个成分，因此需要完整的状语 in October.

Answer 2

when
这是定语从句,做时间状语

Answer 3

前一个是定语从句，修饰时间用关系词when。后一个是强调句，句型是It is/was …that …，这里是介词短语。

Answer 4

中华人民共和国成立于十月
这里是一个强调句，It was ~~~when 的句型