1) 談及一項長期活動或開始一種習(xí)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.
