The sentence "Nous irons _ la plage demain" requires the preposition a (with accent: a) to express movement toward a destination. The verb aller (to go) uses the preposition a before specific places: aller a la plage (to go to the beach), aller a l'ecole (to go to school), aller au marche (to go to the market).
The complete sentence reads: "Nous irons a la plage demain" - "We will go to the beach tomorrow."
The other prepositions do not work here:
par means "through" or "by" and indicates a route or means, not a destination.
en is used before countries (en France), modes of transport (en voiture), or certain fixed expressions, but not before la plage.
dans means "in/inside" and implies being physically within something, not moving toward it.
The rule is straightforward: aller + a + definite article + place for expressing "going to" a specific location.
