began后为什么要用to do的结构

begin doing和begin to do的区别
1）begin后跟to do或doing，含义上没有多少差异。如果谈论一个长期的习惯动作开始养成时，更加常用begin doing。
例如： How old were you when you first began playing the piano?
2）当begin后接表示心理活动的词时，比如： understand，realize，believe， wonder，know，think等动词时，用begin to do。例如： She began to think she was wrong after a second thought. 经过重新考虑后，她开始认为她错了。
3）当begin用于进行时态时，其后跟to do。如： Mary is beginning to do her homework.
4）当主语是物或用于描述天气变化时，其后跟to do。如： The water began to boil. 水开始开了。 It began to get dark before we knew it. 不知不觉地天黑下来了