The imperfect is used for description, talk about a habit. The action is incomplete:
Exemple : When it was cold he did not forget to put his gloves on the mantelpiece.
(Quand il faisait froid il n'oubliait pas de mettre ses gants sur la cheminée.)
Here it is a habit.
For the past tense; action is accurate and complete.
Exemple : He put his gloves on fire home from work.
(Il a mis ses gants sur la cheminée en rentrant du travail.)
He did it once, it happened dated.
