Originally Posted by Fuji
No coach I've worked with has specifically said "oh your elbow is/isn't bent, change that." I think it's just a natural thing.
After recording my strokes I realized that I was hitting straight arm forehands, I never knew or felt a difference before seeing it on camera. But as like many people say about Fed and Nadal, they don't always hit straight arm. The normally straight arm to bent arm forehand happens mainly when you're out of position or when swing timing is bad. My first correlation of this was seeing the distance to contact. Watching myself hit, I would say 8 or 9 out of 10 of my forehands were hit with a straight arm (at least 90% straight), and the ones that were bent arm forehands were because I was too close to the ball (and had to naturally bend my arm to make contact closer to my body). They were caused by being out of position and/or bad swing timing, so it's all one big circle of cause and effect, mainly determined by how far you naturally make contact from your body.