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.