Final answer:
To transform the phrase 'Quel est le problème ?' to use the imparfait tense, you would say 'Quel était le problème ?' The imparfait tense is used to describe ongoing or habitual actions in the past.
Step-by-step explanation:
To transform the phrase 'Quel est le problème ?' to use the imparfait tense, you would say 'Quel était le problème ?' The imparfait tense in French is used to describe ongoing or habitual actions in the past. In this case, we are asking about the problem in the past, so using 'était' (imperfect form of 'être') is appropriate.
Learn more about imparfait tense in French