Translation:You used to believe in the lies he used to tell you.
It was not accepted for me, August 28, 2020. I would like to better understand this use of croire à and this translation. In English, when we say "believe in" we mean a person, system, concept or being. You can believe in God, believe in socialism, believe in your friend (that she will succeed), believe in honesty, etc. I simply believe (or don't believe) lies or stories, I don't believe IN them. A much more natural sentence would be "You used to believe the lies he would tell you" or at least "You used to believe the lies he told you." If this example is meant to teach us about the use of the phrase "croire à" and the use of the imparfait, it's a bad example.
To translate this as "to believe in" is not correct in this context IMHO.
"To believe in" as in "to hold a belief or belief system" is "croire en"; eg to believe in God.
"Croire aux fantômes" or "aux esprits" would be "to believe in" but "croire à une histoire" or "à un mensonge" is "to believe" and NOT "to believe in".