How do you say "Call me at 8 pm,or the earliest time you can do/that's convenient 4 you after that"?
Call me at 8 pm, or the earliest time you can.
Call me at 8 pm, or the earliest time that's convenient for you after that
2) What's the difference in imperative tone between "Think of it" and "Come to think of it"?
"Think of it" - no idea how you would use this.
"Think on it" means Go! Do this! Think about it! Usually used after something needs changing. Implying something changes.
Come to think of it = I've just had a thought.
3) How do you say "It's common for guys to bare their top body in the gym"?
It's common for guys to bare their upper body in the gym. - ok, but a little formal.
It's common for guys to bare their upper bodies in the gym. - ok, but a little formal.
It's common for guys to go bare chested in the gym.