If animation is 40 frames, should I cut it off at 39? If I am making an animation loop that is 40 frames and I start it at 0, do I need to end it at 39 or 40 to make it loop right?



Another way to ask the question might be, does the last frame count as a frame?
In short, yes. Frame 40 does count as a frame, unless you specifically tell the timeline or NLA editor to ignore it.