Learn Italian: Potresti copiare il file nella cartella nuova? - Could you copy the file into the new folder?

Potresti is the conditional form of potere (to be able to), meaning "could you". In Italian, just like in English, using the conditional makes a request much more polite than using the present tense puoi (can you).

Since file is an English word, how do we know it is masculine (il file)?

Foreign loanwords adopted into Italian are almost always assigned the masculine gender by default. You also pronounce it exactly like the English word, not "fee-lay"!

Why does the sentence say nella instead of in la?

In Italian, you cannot put the preposition in directly next to a definite article like la (the). You must combine them into a single word. The combination of in + la always becomes nella.

Why does nuova come after cartella? Could I say nella nuova cartella?

Yes, you could! Nuovo/nuova is one of the few adjectives that can go before or after the noun. Placing it after (cartella nuova) emphasizes the physical newness of the folder. Placing it before (nuova cartella) generally just means it is another, recently created folder.