No sooner had they got off the train _______ it started moving.

no sooner.than 固定用法"一...就..."
引导时间状语从句,主句用过去完成时,"than"后面的从句用一般过去时.(从这里可以知道,no sooner后面是主句;than后面是从句)
例:
I had no sooner reached home than it began to rain.
我一到家天就下起雨来
No sooner ...than...常放在句首,表示强调.这时主句要倒装.
例:No sooner had I reached home than it began to rain.
与这个短语类似的还有 hardly ...when
意思为:"还没...就..."
例:
We had hardly gotten there when it began to rain.
我们还没到那儿,天就下起雨了.
(主句用过去完成时,从句用一般过去时)