my mum doesn't или don't drive very fast Ellen doesn't или don't play the violin They doesn't или don't know how to sing this song.

1. My mum doesn't drive very fast.

2. Ellen doesn't play the violin.

3 They don't know how to sing this song.

Doesn't Ставится когда объект отвечает на он, она, оно (He, she, It).

Don't ставится когда объект отвечает на они, я, мы, ты, вы (They, I, we, you)