1.he is a great cook.he is/can cook really well. 2.look! mary is dance/dancing. 3.are you/can you ride a bike? 4.harry and tom aren't/isn't sleeping.they're in the garden.

1 - can, 2 - dancing, 3 - can, 4 - aren't

1. He is a great cook. He can cook really well.
2. Look! Mary is dancing.
3. Can you ride a bike?
4. Harry and Tom aren't sleeping. They're in the garden.