The two Italian verbs sapere and conoscere are often both translated as "to know". But they have very different uses. Sapere is used with facts, data, specific pieces of information, names, phone numbers and addresses. You will often see it followed by words that look like interrogatives such as quando, quanto, come, perché, etc. When followed by an infinitive (dictionary form of a verb), it means "to know how to".
Non so quando arriva il treno. - I don't know when the train arrives.
Sai il nome del professore di matematica? - Do you know the name of the math prof?
Non so ballare. - I don't know how to dance.
The Italian for "I don't know" is Non lo so. English drops the object pronoun, Italian keeps it (lo).
Conoscere carries the sense of "to be familiar with," "to be acquainted with," "to meet" or "to get to know." You will often see it followed by a noun or object pronoun.
Conosci questo ristorante? - Are you familiar with this restaurant?
Vorrei conoscere tua sorella. - I'd like to meet (or get to know) your sister.
Conosco quella ragazza, ma non so il suo nome. - I know that girl, but I don't know her name.
In the past, sapere means "to find out" and conoscere means "to meet."
Ho saputo che Mario sta male. - I found out Mario is sick.
Ho conosciuto una ragazza simpatica a Firenze. - I met a nice girl in Florence.
lui / lei sa
lui / lei conosce