"He played the banjo but he forgot."
Translation:Sheinn sé an bainseó ach rinne sé dearmad.
Yes; it would require the imperfect (past habitual), taught near the end of the tree. The way I interpreted this sentence is: "He (had) played the banjo (once before, perhaps a long time ago) but he forgot (he had played it before)". So, maybe he told someone he hadn't played before when he had, because he had forgotten it... I hope that makes sense.