Learn French: Nous sommes censés écouter l'hymne en silence. - We are supposed to listen to the anthem in silence.

Because censé functions as an adjective in this structure, so it must agree in gender and number with the subject it modifies. Since the subject is Nous (plural), we add an s. If Nous referred exclusively to a group of women, it would be censées.

Can I say Nous sommes supposés instead of Nous sommes censés?

In strict French, no. Être censé is the correct phrase for obligations or rules (what you are expected or meant to do). Être supposé is traditionally reserved for hypotheses or assumptions (what is presumed to be true). However, due to English influence, you might hear native speakers use être supposé for obligations in very casual speech. Stick to être censé to sound natural and grammatically correct.

Why is there no French equivalent for the word "to" after écouter?

In French, écouter is a transitive verb, meaning it connects directly to its object without needing a preposition. The idea of listening to something is already built into the verb itself. Therefore, it is always écouter l'hymne, never écouter à l'hymne.

Why do we use en silence instead of dans le silence?

En silence is a set phrase used to describe the manner in which an action is performed (essentially meaning "silently"). Dans le silence is used more literally to describe an environment or physical state, such as working in a completely quiet room.