"My son will be a policeman."

Translation: Mój syn zostanie policjantem.

[jaroze]

How can you tell whether 'zostać' means 'to become' versus 'to stay' in a given context?

[Vengir]

„Zostać” as in "to become" always comes with an object noun in instrumental case.

[jaroze]

And what is the case that accompanies 'to stay'? Like if you were to stay a police officer?
Thanks!

[Vengir]

Ah, you meant that kind of "stay", when a person is already a policeman and will continue to do so? Then I would suggest using verbs like "pozostać", or "nadal być".

[jaroze]

Okay that's good to know, thanks!