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.