8.11 begin(start) doing/to do
 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?
2) begin, start用进行时时,后面动词用不定式to do
  I was beginning to get angry。
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。
4) 物作主语时
  It began to melt.