I would have told him the answer had it been possible这是一句虚拟语句。 had it been possible是“if it had been possible"的倒装省略句 but I was so busy then则是一般过去式,是真实的,确实发生了的事情。所以用was
语法归属:情态动词+have done sth 表示对过去的事情(也就是已经发生了的事)推测、假设等 例: could /would have done 本来能够做某事/原想做某事 should have done 本应该做某事,而实际没有做 need have done 本需要做某事而没有做 may have done 可能已经做过...