A treadmill is a machine that lets you run or walk without going anywhere. Most people today use treadmills for exercise indoors. A long time ago, treadmills would have been used to power machinery.
Twill is a pattern of alternating diagonal lines woven using cotton or wool. Twill is often used to make coats and sweaters.
You use the word will when you are talking about something that should happen or is about to happen. Using will often implies some force, especially if you are giving an order. Some examples are: "You will clean your room" "I will see you tomorrow" "You will stop doing that"
A windmill is a machine that is used to convert the energy from the wind blowing past into more useful energy. Windmills have several blades or sails that catch the wind as it goes past, and the sails turn around. Inside the windmill is other machinery that does useful work as the wind turns the sails outside. Windmills were used to mill grain into flour, hence their name. Windmills can also be used to pump water out of wells. More recently wind has been used to generate electricity. The version of the windmill that is used for this is called a wind turbine.