不一定。没有这个规定。
原句中可以包含好几个时态,因此改成强调句后仍然可以有多个时态。强调句不改变原句时态。
例如:It is the piano that he was playing when I arrived there.
强调句的时态:如果强调句为现在时则用“It is…that/who…”;如果强调句为过去时则用“It was…that / who…”.
例. It is my mother who often helps me with my homework.
It was last night that I saw the car.
时态要保持一致。
表示过去的就用 it was .....that