"I do not know his number."

Translation: Nie znam jego numeru.

[sliwinska]

Why "Nie wiem" doesn't work here?

[mihxal]

You can't say "Nie wiem jego numeru". You should say "Nie wiem, jaki jest jego numer" or similarly.

[Jellei]

As the English wiktionary says:

znać - to know somebody/something (be acquainted or familiar with)

wiedzieć - to know (be certain or sure about something)

[immery]

While this is true , when you look closely you will find holes in this theory.

the difference between znać i wiedzieć is also in what follows them .

Znać + kogoś/coś ( a noun in accusative)

wiedzieć+ a phrase or about sth

[pacecarotepatate]

Sometimes liczba is listed as sinonym with numer. Is this not true when talking about a phone number?

[Jellei]

Both are translated as "number", but:

liczba is an abstract entity in mathematics, it's the grammatical number, and the amount of something countable.

numer is "used to show the rank of something in a list or sequence", so it's a house/flat/room number, a number on your locker, on a footballer's shirt, also a telephone number.

Technically, without a clear context, "liczby" could be okay here... but maybe let's stick to "numeru".

[pacecarotepatate]

ok, understood. thanks have a lingot

[J_Rzesiewicz]

What is cyfra then?

[Jellei]

A digit. 1/2/3/4/5/6/7/8/9/0.

[Galois33]

Why is numeru in the genitive?

[alik1989]

Because it's a direct object in a negated sentence.

[BartoszKod1]

Dlazcego nie akceptuje: Nie znam jego liczby???

[Jellei]

Czym miałaby być "jego liczba"? Nawet jeśli technicznie rzecz biorąc jest to poprawne tłumaczenie, myślę że ze względu na bardzo ograniczony kontekst w którym byłoby poprawne lepiej jest je odrzucić. Na 99% chodzi tu o numer telefonu, ewentualnie 'numerek' jak w kolejce na poczcie.