"You are borrowing a phone from me."
Translation:Pożyczasz ode mnie telefon.
Yes, you are right about which is which. As for the first question... you don't. English is ambiguous, and Duolingo sentences are just something that's taken out of context. So in 99% of the sentences, both singular 'you' and plural 'you' are equally probable and will both be starred answers. And in 1% or less something else in the sentence will specify it.
Borrow from is not a phrasal verb. It would also be correct to say: "You are borrowing the phone". The verb will still have the same meaning, but the sentence will lack the information who you borrowed it from.
Pożyczać is ambigious the same way одалживать is. If you want to make it clear that you are in fact borrowing and not lending, you must add od(e) + (provider of the item).
In Russian you would basically do the same thing: Ты одалживаешь у меня телефон.