As soon as the teacher left the classroom,the students start

C
begin / start to do sth
begin / start doing sth.
1) 谈及一项长期活动或开始一种习惯时,使用doing.
How old were you when you first started playing the piano?
你几岁时开始弹钢琴?
2) begin,start用进行时时,后面动词用不定式to do
I was beginning to get angry.
我开始生起气来.
3) 在attempt,intend,begin,start 后接know,understand,realize这类动词时,常用不定式to do.
I begin to understand the truth.
我开始明白真相.
4) 物作主语时
It began to melt