Sapere or conoscere? The difference in Italian

The grammatical test, which is the most reliable

Look at what follows the verb. Sapere takes a subordinate clause (so che è partito, so dove abita, non so se verrà) or an infinitive (so nuotare*, so guidare*).

Conoscere only ever takes a noun, never a clause or an infinitive: conosco Marco*, conosco Roma, conosco quella canzone. If only a noun can follow, the verb is conoscere*.

This is the fastest way to decide in real time, because it doesn't require thinking about meaning — just look at the shape of the sentence.

Where they look interchangeable, and aren't

So l'italiano and conosco l'italiano are both correct but say different things: the first means you speak it, the second that you're familiar with it — you'd use it about a language you study but don't speak.

With people conoscere is the only option: conosco Marco works, so Marco doesn't. But so chi è Marco works, because there's a clause there and sapere returns.

Places behave the same: conosco Roma (I've been, I can find my way) versus so dov'è Roma (I have the information).

The past tense changes what sapere means

As with dovere and potere, the tense shifts the sense: *sapevo che era partito means you already had the information; ho saputo che era partito* means you found it out at that moment.

Conoscere behaves the same: *conoscevo Marco (I'd known him a while) versus ho conosciuto Marco* (I met him). It's the same mechanism that governs passato prossimo versus imperfetto.