Here are the rewritten sentences as requested:

1. The girl worked hard. Her stepmother wasn't happy.
-> Although the girl worked hard, her stepmother wasn't happy.

2. Would you please change this T-shirt for me?
-> Do you mind changing this T-shirt for me?

3. Jill doesn't run as fast as Tom.
-> Tom runs faster than Jill.

4. Keeping the environment clean is very important.
-> It is very important to keep the environment clean.

5. A sport car goes faster than an ordinary car.
-> An ordinary car does not go as fast as a sport car.