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?
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.
8.12 感官动词 + doing/to do
感官动词 see, watch, observe, notice, look at, hear, listen to, smell, taste, feel + do 表示动作的完整性，真实性;+doing 表示动作的连续性，进行性
I saw him work in the garden yesterday.
昨天我看见他在花园里干活了。(强调"我看见了"这个事实)
I saw him working in the garden yesterday.(强调"我见他正干活"这个动作)
昨天我见他正在花园里干活。
典型例题
1)They knew her very well. They had seen her ___ up from childhood.
A. grow B. grew C. was growing D. to grow
答案：A。因题意为，他们看着她长大，因此强调的是成长的过程，而非正在长的动作，因此用see sb do sth 的句型。
2)The missing boy was last seen ___ near the river.
A. playing B. to be playing C. play D. to play
答案A. 本题强调其动作，正在河边玩，应此用see sb. doing sth句型。