"Balanced diet is to provides us with nutrition which develops our body to repair decay of our body & gives strength to our body." 聼起來自然嗎?

1. The verb right after "to" needs to be in its base form. This is because of to-infinitive grammar rule.
Reference: https://learnenglish.britishcouncil.org/en/engl...
http://www.ef.com/english-resources/english-gra...

2. "which develops our body to repair decay of our body..."
That part doesn't make sense at all (although it's grammatically correct). How does "developing your body" helps to repair...? Developing your body is developing your body, repairing is repairing. There is no link.
If you want to make a link, you've to link it. i.e. which develops our body the ability to repair damaged cells and gives us strength.

3. Try not to repeat any words in a sentence.