I'm level 69 and I've been doing most of the side quests that I can. I've been trying to do the companions' quests and I'm stuck. I went to the forsaken cave and killed the draugr but when I got to the leader of the forsaken cave he was already dead. I Sarana with me, does that make a difference? Help!

More than likely, the only fix for this issue will be to reload an earlier save before accepting that quest. Having Serana with you shouldn't have anything to do with this bug occurring

If you are playing the game on the PC (by chance) and the leader's corpse is still there, you may be able to get the quest to complete by clicking on the corpse with the "console" open and typing "resurrect 1" (without the quotes). If this works, then killing him "should" allow the quest to update.