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Quick question on Polish language fundamental [40]
Are there any more rules for using the Instrumental case as i'm slightly confused now?
Seriously, don't worry too much about it.
The main uses are: 1) instrument, 2) być, 3) prepositions
You'll often find the instrumental replaces a word in English, typically "with" or "by," but with "być", it usually replaces the indefinite article. And as gumishu said, it can replace other words, such as "wieczorem" = "in the evening."
Instead of thinking of it as a big, scary, Slavic case, think of it as shorthand, and something that makes your life easier, because you can use fewer words, don't have to remember so many prepositions, and you don't have to worry about word order ;)
"Jestem AKTORKĄ" = "I am AN ACTRESS"
Why instrumental and not nominative? Well, Polish loves having a flexible word order, and case marking means you can switch things around for emphasis and still be completely correct. "Aktorką jestem" is 100% natural and correct Polish too.
"Interesuję się FILMEM" = "I am interested IN FILM" (literally "I am interested with/by film")
"FILMEM się interesuję" = "I am interested IN FILM"
As for prepositions, "z" doesn't mean much until you combine it with either the instrumental or genitive case.
z czymś = together with something
z czegoś = out of something
z serem (inst) = together with cheese
kanapka z serem = sandwich with cheese = cheese sandwich
z sera (gen) = out of cheese
sos z sera pleśniowego = sauce made out of blue cheese = blue cheese sauce