Here are the corrections:

1. **Incorrect**: You should wash your hand before meals.
**Correct**: You should wash your hands before meals.

2. **Incorrect**: There are some milk to drink.
**Correct**: There is some milk to drink.

3. **Incorrect**: Walking is gooder than travelling by car.
**Correct**: Walking is better than travelling by car.

4. **Incorrect**: She have a round face.
**Correct**: She has a round face.