Learn Portuguese (Portugal): Nós sentamo-nos no sofá.

When the reflexive pronoun nos is attached to the end of a verb conjugated for nós (which ends in an 's', like sentamos), we drop the final 's' of the verb to make it easier to pronounce. So, sentamos + nos becomes sentamo-nos.

Why do we need the reflexive pronoun nos here? English just says 'we sit down'.

In Portuguese, the base verb sentar actually means 'to seat' (someone else). To say that you sit yourself down, you have to use it as a reflexive verb: sentar-se (literally 'to seat oneself'). Since the subject is nós (we), the reflexive pronoun must match it, becoming nos (ourselves).

Why is the pronoun nos attached to the end of the verb with a hyphen?

In European Portuguese, reflexive and object pronouns are normally attached to the end of the verb with a hyphen in simple, affirmative sentences like this one. If the sentence were negative, a word like não would pull the pronoun to the front, before the verb: Nós não nos sentamos.

Do we have to include the word nós at the start of the sentence?