my mum likes to watch me ------ the piano

play
watch 后作宾语补足语的动词不定式要去掉to!
i often watch him plau football
我经常看他打球
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,+doing 表示动作的进行性.例如：
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.