Prepositions

In French, the preposition “avec” is used to express being or doing something together with someone.
The verb “s’asseoir” (to sit) can be followed by “avec” to mean “to sit with” a person or group.
The context shows that the speaker is refusing to sit with certain people, so “avec” is appropriate.
Examples:
— Je veux m’asseoir avec mes amis. (I want to sit with my friends.)
— Il s’est assis avec ses collègues. (He sat with his colleagues.)
The negative structure “ne... pas” wraps around the verb and its pronoun: “Je ne veux pas m’asseoir...”
So the complete sentence becomes: Je ne veux pas m’asseoir avec ces personnes.

When the verb “commencer” is followed by another verb in the infinitive form, French grammar requires the preposition “à”.
The structure is: commencer à + infinitif.
In English, this means “to begin to do something.”
Examples:
— Il commence à pleuvoir. (It is starting to rain.)
— Nous avons commencé à lire ce livre hier. (We started reading that book yesterday.)
Using “de” instead of “à” would be incorrect in this case, because “commencer” takes “à” before verbs.
There are some verbs in French that take “de” (e.g., “essayer de,” “arrêter de”) and others that take “à” (e.g., “aider à,” “réussir à,” “commencer à”).
Thus, the correct structure is: Ils commencent à courir. (They begin to run.)

The verb monter means “to climb” or “to go up,” but in the context of transportation, it means “to get into” a vehicle.
In French, when referring to getting inside a vehicle (car, train, bus, plane), the correct preposition to use is “dans”, which indicates entering a closed space.
It’s different from “sur”, which would be used for open surfaces (like “monter sur une moto” — to get on a motorcycle).
Let’s look at similar examples:
— Il est monté dans l’autobus. (He got into the bus.)
— Elle est montée dans l’avion. (She got into the airplane.)
The noun “le train” (the train) is a closed space, so we must use “dans.”
Incorrect: monter sur le train (This would imply climbing on top of the train!)
Correct: monter dans le train (to board the train normally)
Therefore, the correct sentence is: Nous sommes montés dans le train.