These few tips have worked over and over for me, am sure it won't fail you.
#To harden soft nails, soak them in warm olive oil for about 20 minutes, on alternate days.
Pour the oil into a bowl. Dip that bowl into hot water as shown, then soak your finger nails into the warm oil.#Whenever you wash your hands, while bathing as well as otherwise,
make sure to dry the nails properly and apply a hand cream or lotion, using it on the nails as well. This is because soap makes the nails dry and brittle. 
All these won't work if u keep biting your nails so STOP!