"Elle est dans l'avion."
Traduction :She is on the plane.
On peut dire "in the plane, in the train, …" mais l'usage veut que l'on utilise "on"
"Francis B You can't try to make sense of idiom. We can be on or in an airplane, but the ordinary usage is "on", the same way we are on a bus or on a train, meaning we are traveling by means of it en masse. But, say there is a hostage situation, and the plane is sitting on the runway while the skyjackers decide what their demands are. The chief of security would probably want to know how many people were in the airplane. They would also be "on" it in that case, but "in" becomes possible when you look at the airplane as a container and not a vehicle. The other answerers got a little mixed up, I'm afraid.
"To take part in" is a phrasal verb. The "in" is indispensable. There is no verb "to take part on".
It's not "in THE bed", it's "in bed", meaning disposed for sleep or whatever you intend to do in the bed. You can be "on the bed", meaning situated on top of it, like sitting there, but that clearly means you are not in bed. You can't be "on bed"."