4.  It + be + 时间 + that-clause

这个句型是个强调句型。例如：

It was two years ago that he made an important invention .

(原句是：He made an important invention two years ago . )

It was at 5 o'clock that he practised playing the violin in the morning .

(原句是：He practised playing the violin at 5 o'clock in the morning . )

比较：It was 5 o'clock when he started in the morning .

(5 o'clock前没有介词，这是个定语从句)