Learn Polish: Będąc młodym astronautą, musisz dużo pracować. - Being a young astronaut, you have to work a lot.

Będąc is the adverbial contemporary participle (meaning "being") of the verb być (to be). You generally form these by taking the third-person plural present tense of an imperfective verb, but być is slightly irregular because we build it from the future tense oni będą (they will be). We drop the final -ą and add the suffix -ąc. These participles express a background action happening at the exact same time as the main verb.

Why is młodym astronautą in the Instrumental case here?

Because the participle będąc keeps the grammar rules of the verb it comes from, być (to be). In Polish, when you state that someone is a noun (like a profession, role, or identity), that noun and its adjectives take the Instrumental case. So, just as you would say jesteś młodym astronautą (you are a young astronaut), you must also use the Instrumental case after będąc.

How do we know who is the young astronaut in this sentence?

A strict rule of the -ąc participle is that the "doer" (the subject) of both parts of the sentence must be exactly the same. Since the main clause uses the verb musisz (you must), the implied subject is "you". Therefore, "you" must also be the one who is the young astronaut. You cannot use this participle form if the two actions are done by different people.

Can I just say Kiedy jesteś młodym astronautą instead of using będąc?