Biting your fingernails is a bad habit.
- Ronger tes ongles est une mauvaise habitude.
You have dirt under your fingernails.
- Tu as de la crasse sous les ongles.
I never cut my nails at night.
- Je ne me coupe jamais les ongles le soir.
He managed to cure himself of his habit of nibbling his nails.
- Il a réussi à se guérir de son habitude de se ronger les ongles.