I also just started learning Portuguese, so I'm sure other users will post more competent replies, but here the "a" is not an article, but a preposition. You have to learn the verb like this "assistir a". This preposition is just not always translated into English. Os alunos assistem às aulas. The students attend the classes. (i.e. are present AT the classes)

Hi Ramon. It's a hard question even for native speakers. They always misuse this verb. Actually, the verb assistir admits two possibilities one with the preposition a and another without the preposition. "Assistir a" means watch and assistir without a means help or assist. So, because of that you see "eu assisto ao espetáculo" e "o médico assiste o paciente". I hope I could help.

Hi, Ramon! The word "a" in Portuguese can be an article or a preposition. In this case, it is a preposition, that's why it is not the same as saying "The father and the son are watching the a movie". The word "a" is needed because of the verb "assistir", which requires this preposition when meaning "watch". If you didn't use this preposition after this verb, the sentence would have a whole different meaning, once that "assistir" would be the same as "help". In Portuguese, some verbs can mean different things depending on the following preposition, you can get to know more about it by studying "regência verbal". I am also on your disposal to explain any doubts you might have.

O nome disso é regência verbal. Mas a gente normalmente no dia a dia não segue essa regra a risca. A gente falaria Eles estão assistindo um filme. Mas está gramaticalmente errado.

Assistir a = observar; presenciar Assistir em=morar em Assitir alguém = dar assistência; ajudar