It is high time that they started setting off on a trip ,这句话中为什么要用started ,

It is high time that they started setting off on a trip ,这句话中为什么要用started ,
It is high time that they started setting off on a trip ,这句话中为什么要用started ,

It is high time that they started setting off on a trip ,这句话中为什么要用started ,
这是固定用法
it is (high) time that someone did something.

it is high time that did
这个是强调句句型

虚拟语气：It is (high) time that 后面的从句谓语动词要用过去式或用should加动词原形，但should不可省略。
　 It is time that the children went to bed.
　 It is high time that the children should go to bed.

It's (high/about) time+ that--从句 从句用一般过去时，虚拟语气。如：
你该上学了。
It's high/ about time that you went to school.=It's time for you to go to school.