begin(start) doing/to do
　　
begin/start to do sth
begin/start doing sth.
　　
1)談及一項長期活動或開始一種習慣時，使用doing.
How old were you when you first started playing the piano?
你幾歲時開始彈鋼琴？
　　

2)begin, start用進行時時，后面動詞用不定式to do
I was beginning to get angry。
我開始生起氣來。
　　
3)在attempt, intend, begin, start 后接know, understand, realize這類動詞時，常用不定式to do。
I begin to understand the truth。
我開始明白真相。
　　
4)物作主語時
It began to melt