Re: A or The and punctuation
I'd change these:
If the basket and the envelope are likened to the car's main body and the wheels the main body and wheels of a car , another part, the burner, would be a the core engine that provides driving force to the wheels. While the envelope and the basket are the most visible parts of the balloon, the burner positioned under the envelope is not easily visible from outside, but it plays one of the most important roles in the balloon flight.
I'd omit the two words I've underlined.