你的问题很简单
1 是由主动语态变来的:Someone saw the missing boy playing near the river.
see sb.doing:看见某人在做某事
see sb.do sth看见某人做了某事
2 因为someone看见他的时候,the boy正在玩,所以就成了saw him playing
3 再把Someone saw the missing boy playing near the river.改写成被动语态:
The missing boy was last seen playing near the river.改写成被动语态时,把原来的宾语变成主语,在把saw改写成was seen,后面的playing不变.现在,你明白了吗?
