* Do you enjoy this exercise?

— Your friend likes to exercise. In particular, he likes press-ups. One day, you're watching him do press-ups. He has reached 756. Sweat pours from his brow. He gasps for breath. The muscle strain is audible. It seems remarkable that anyone should voluntarily endure such discomfort. At last you can restrain yourself no longer. "Tell me," you say, "do you enjoy this exercise?" (At that moment he expires. You never learn the answer to your question.)

* I look forward to receiving your reply.

— Fine. Cf. "I look forward to hearing from you": very common at the end of a letter.

* When do you come back?

— Fine. Just as common, in BrE: "When are you back?"

MrP