1) 談及一項長期活動或開始一種習慣時，使用doing。

　　How old were you when you first started playing the piano? 你幾歲時開始彈鋼琴?

　　2) begin, start用進行時時，后面動詞用不定式to do

　　I was beginning to get angry.　我開始生起氣來。

　　3) 在attempt, intend, begin, start 后接know, understand, realize這類動詞時，常用不定式to do。

　　I begin to understand the truth.　我開始明白真相。

　　4) 物作主語時

　　It began to melt。

　　感官動詞 + doing/to do

　　感官動詞 see, watch, observe, notice, look at, hear, listen to, smell, taste, feel + do　 表示動作的完整性，真實性;+doing 表示動作的連續性，進行性

　　I saw him work in the garden yesterday.　昨天我看見他在花園里干活了。(強調"我看見了"這個事實)

　　I saw him working in the garden yesterday. (強調"我見他正干活"這個動作)昨天我見他正在花園里干活。

　　典型例題

　　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. 因題意為，他們看著她長大，因此強調的是成長的過程，而非正在長的動作，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. 本題強調其動作，正在河邊玩，應此用see sb. doing sth句型。