Dude!!!! Can you help me with the problem of cats in my neighbourhood? Yeah, I know a dog
No, you must use the accusative case because the dog is the direct object of the sentence.
Is "I know that dog" really wrong but "I know the dog" correct? I know there is no tamten but I though either would work? Or is that a beginner mistake?
I've seen this happen a few times to a few different people, and here is the explanation: Polish has no words for any articles. Articles being a, an, and the. Polish does have words for demonstrative pronouns though, being this, that, these, and those. To say "I know THE dog" would be "Znam psa," notice how there is one conjugated verb and psa is in the accusitive, there are only two words. There are only two because the pronoun "I" is part of the verb "Znam," and the word "the" does not exist in Polish. To say what you want to, I know THAT dog, would be "Znam TEGO psa," we add tego because Polish does have a word for "that," and it's in the accusative. There are three words, "I" is part of the conjugated verb, "tego" is now here because Polish has a word for that, and psa is the direct object, in the accusative.
Just to add: technically the direct translation of "I know that dog" is "Znam tamtego psa", but "Znam tego psa" is perfectly acceptable because Polish and English think differently about the 'distance' (this/that/that = tego/tego/tamtego).
Well, not much more to say than what I wrote already (this/that/that = tego/tego/tamtego and their forms), but you can read here: https://www.clozemaster.com/blog/polish-demonstrative-pronouns/