Nous irons _ la plage demain.

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.