She watched the kids playing in the yard. 指观看的时候“玩”的动作正在进行，仅指当时一时的情况，至于“玩”的动作何时开始何时结束都不得而知。
They watched the bus disappear into the distance.指观看到巴士消失的整个过程，从看到它一直到它消失在远方。
比较：
I watched him repairing the washing machine. （仅指修理的动作正在进行）
I watched him repair the washing machine. （修理的全过程）