Good day,

I'm reading Le Petit Prince, and I stumbled upon a statement in chapter 5 that goes "ce fut grâce au mouton, car brusquement le petit prince m'interrogea, comme pris d'un doute grave."

I don't know why "fut" was used instead of "était" and "m'interrogea" indicates something in the past, however, I cannot see the forms used in imparfait or passé simple. What kind of form is fut, courus and interrogea? And how to use them?

Thanks in advance.