Learn Portuguese (Portugal): Nós vemos um vídeo.

Actually, this sentence is in the present tense! Vemos means 'we watch' or 'we see'. If you wanted to say 'we watched' in the past tense, you would use vimos (Nós vimos um vídeo). It is helpful to contrast the present and the past so you do not mix them up!

Why do we use the verb ver (to see) for 'watch'?

In English, we make a strong distinction between 'seeing' and 'watching', but in Portuguese, it is completely normal to use ver for watching TV, movies, or videos. There is another verb for 'to watch' (assistir), but ver is much more common in everyday spoken Portuguese.

Can I drop the word Nós and just say Vemos um vídeo?

Yes, absolutely! Because the verb ending -mos is only ever used for 'we', the pronoun Nós is optional. Portuguese speakers often drop it unless they want to add emphasis, like saying 'WE are watching a video (not them)'.

Why is there an accent mark on vídeo?

The acute accent (´) on the í tells you to put the spoken stress on that first syllable. Without it, the default pronunciation rules would force the spoken stress onto the second-to-last vowel (the e).