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。