基础语法知识：since的四种用法
1) since +过去一个时间点(如具体的年、月、日期、钟点、1980, last month, half past six)。
I have been here since 1989.
2) since +一段时间+ ago
I have been here since five months ago.
3) since +从句
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.
4) It is +一段时间+ since从句
It is two years since I became a postgraduate student.
基础语法知识：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 playingC. playD. 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。
往期知识要点汇总
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