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。

　　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。