是个固定短语 start to do 或者start doing
&区别在于：
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 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.
一般情况下没区别
只有三种情况只用start to do sth.
1.start本身用-ing
He is just starting to write the letter.
2.主语是物不是人
The ice started to melt.
3.其后的动词与想法\感情有关
She started to understand it.
