Learn Portuguese (Portugal): No recreio os alunos falam.

In English, we say "at break", which might make you want to use the preposition a (at/to) and its contraction ao. However, in Portuguese, when talking about events or being located somewhere during a specific time, we usually use em (in/on/at). The preposition em combines with the masculine article o (the) to make no (literally "in the").

How do we know falam means "they talk"?

The verb is falar (to talk/speak). For regular verbs ending in -ar, the "they" form (third-person plural) always ends in -am. This correctly matches the subject of the sentence, os alunos (the students).

Does os alunos only refer to male students?

Not necessarily. In Portuguese, the masculine plural os alunos is used for a group of all boys, but it is also the default form used for any mixed group of male and female students. You would only use as alunas for an all-girls group.

Why is there an os before alunos?