Prepositions a and em

I’m struggling to remember when to use the preposition a and when to use em. I know it’s largely a matter of memorising enough examples so it becomes “easy” to “automatically” choose the correct one. Today I had three examples in my smart review

  • nós visitamos a nossa mãe aos domingos
  • nós vamos ao cinema à noite
  • nós trabalhamos no sabado

I cannot find a way to remember that it has to be a for the first example but em for the last one. Is there any logic to this? Or are they more interchangeable so it doesn’t matter so much?

Edited a day later:

Having thought more about it, I “jumped to the conclusion” that example one and example three should use the same preposition, because both were about something happening on a specific day.

But is it maybe rather the verb that determines it? So that example one and two both use a simply because that is the preposition used for verbs visitar and ir, whereas example three uses em just because that is the suitable preposition for trabalhar?

That would be similiar to how, for instance, liking you is gostar de ti, but thinking about someone is pensar em ti, even though the object is ti in both sentences.

And as a follow-up from today’s smart review

  • ela trabalha esta temporada

Why was the preposition em needed for nós trabalhamos no sabado, but not for ela trabalha esta temporada?

It’s always hard to give guidance on the use of prepositions. In any case, what I can explain in terms of patterns is that:

  • Prepositions are optional with the demonstrative determiner este/esta. So, sentences such as “Ela trabalha esta temporada” or “Vou lá este domingo” sound perfectly natural for us. The same happens in English, for example. We say “I’ll be there this Sunday”, not “…on this Sunday”.
  • In reference to days of the week, pluralized days tend to go with the preposition a (hence “aos domingos”), while singular days might go with a, em or nothing. “Nós trabalhamos no sábado”, “Nós trabalhamos ao sábado” and “Nós trabalhamos sábado” are all acceptable sentences.
  • For periods of the day… Mornings always take the preposition de. Afternoons and evenings might take both de and a. If we want to specifically say this afternoon or tonight, we should only use a. If we’re talking more generally, e.g. in the afternoon or at night, we can use either.

For these examples in particular, the verb is not a factor.


Grammar is a beautiful thing, but it’s not always easy! Thank you very much for the guidance. I’ve made a note of the guidance and will go over it repeatedly until it starts to “stick”.

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