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，後面動詞用不定式to do
i was beginning to get angry。
我開始生起氣來。
3) 在attempt, intend, begin, start 後接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.