My nails are thin and not hard. What can I do in terms of dieting or other methods that can make them harder and stronger?

Apply vitamin E oil. And make sure you take a vitamin b complex supplement. Otherwise, maybe have your thyroid checked.
Various causes. Nail abnormalities can sometimes be a clue to an underlying systemic illness or disease. Getting your nails "done" to include polish and fake nails can all potentially damage your nails. You should probably have your regular doctor screen you for things like anemia, thyroid problems and others. A dermatologist eval may also be necessary.