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。