Prepositions for parts of the day
When you talk about parts of the day in English, you switch between prepositions depending on the word. You do things in the morning, but you sleep at night. Portuguese also uses specific prepositions for parts of the day, but they do not match the English ones mechanically.
Instead of a single word for in or at, Portuguese relies on fixed combinations of the prepositions de and a to express when something happens.
The three most common parts of the day are manhã, tarde, and noite. To say that something happens in the morning, Portuguese uses the preposition de to form de manhã.
However, for the afternoon and evening, it is standard to use the preposition a combined with the feminine article a. This creates the contraction à, giving us à tarde and à noite.
When specifying exact turning points like midday (meio-dia) and midnight (meia-noite), Portuguese uses the preposition a with the definite article.
Since meio-dia is a masculine noun, it becomes ao meio-dia. For the feminine meia-noite, you use à meia-noite.
Portuguese also has a specific word for the early hours of the morning before sunrise: madrugada. To say something happens during these early hours, you go back to the preposition de. Similarly, if you want to specify that something happens during the daytime, you use dia with the same preposition.
When you talk about tonight, you do not need an extra preposition. The time phrases esta noite or hoje à noite already contain the necessary context to stand alone in a sentence.
Finally, if you are giving a specific time on the clock, the prepositions change slightly. Instead of saying at three in the afternoon, Portuguese phrases it as at three of the afternoon. You will combine de with the definite article, resulting in da manhã, da tarde, or da noite.
Portuguese uses fixed prepositions to talk about parts of the day. Remember to use de for the morning, the daytime, and the early hours, and à or ao for the afternoon, the evening, noon, and midnight.