I used to bite my fingernails when I was a baby.But now I don't bite my fingernails.

To use is a verb meaning of course to utilise or make use of something, example, "I use my bike every day". If you are used to something it means you are very familiar with it, or with doing some activity, example, "He is very used to playing cards on Fridays" or "He is used to walking his dog every day". In the case of a person, "He is very used to his colleague at work". Also, "Used to" expresses something in the past but which stopped and is no longer the case. Example... He used to play the piano but now he plays the guitar.