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.