1. She is talking with her friend in a boat/on a boat. (Which preposition should we use?

Question

2. They are jumping from the ground.
3. They are jumping on the ground.
(Which preposition do we have to use? Are both OK?)

Answer 1

1. "on"

2 and 3 -- both are OK, depending on what is meant.
2 means they are jumping upward (from).
3 means they are jumping downward (on, as if from above).
or 3 means they are jumping up and down on the ground, not on a trampoline or anything like that.