Translation:You used to often come to this restaurant.
You often came..should be accepted as the 'often' tells you it was ona regular basis, so the imperfect tense is implied. The English simple present is as acceptable as 'you used to'.. And is otherwise accepted as a translation of the French imperfect tense in this section. So, I still think Duo is wrong not to accept..'you often came ..'!
This is not specific to any particular verb; it has to do with the tense being used. The use of the imperfect tense is one of several past tenses in French. It is used in the following ways:
- An action that was going on in the past at the same time as another action. The first action is expressed in the imperfect and is usually translated "was + present participle", e.g., Il lisait un livre pendant que elle écrivait une lettre = He was reading a book while she was writing a letter. Both actions were taking place in the past.
- An action that was going on in the past when another action occurred: Nous mangions le dîner quand ils sont arrivés = We were eating dinner when they arrived. Note: Passé composé is used for the second action.
- An action that a person did habitually in the past, e.g., Les enfants allaient à la plage tous les jours = The children used to go to the beach every day. Without reference to another past action, the use of the imperfect tense compels the interpretation of a habitual action, so they "used to go...." which carries the sense of the habitual action into the English.
- A description of a mental or physical condition in the past, e.g., Elle n'était pas malade quand je l'ai vue. = She was not sick when I saw her.
- An action or state of being that occurred in the past and lasted for a certain length of time prior to another past action, e.g., J'attendais l'autobus depuis dix minutes quand il est arrivé = I waited for the bus for ten minutes when it arrived.
- The imperfect tense can also be used to "set a scene", e.g., C'était une soirée tranquille = It was a peaceful evening.
- The imperfect tense is used to tell the time of day or express age in the past, e.g., Il était sept heures et demie = It was seven-thirty. (or) Elle avait dix-huit ans = She was 18 years old.