begin / start to do sth

begin / start doing sth.

例句：

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.