Imperative mood (command, order or request) :
Don't eat french fries so often! (subject pronoun "you" omitted)
A direct speech your father always says to you:
My father always says, "don't eat french fries so often."
When converting to an indirect speech involving imperative mood, we use to-infinitive positive sentence or not to-infinitive for negative sentence. (that-clause is only used for indicative mood only!!!!)
My father always tells us/me not to eat french fries so often. (us or me must be added to indicate the omitted you)
positive sentence: My father always tells me to do exercise regularly.