Learn Polish: Dobrze znam ten szkodliwy nałóg. - I know this harmful addiction well.

In Polish, znać (which conjugates to znam here) is used when you are acquainted with a person, place, or thing, like an addiction. You use wiedzieć (wiem) when you know a fact or a piece of information, usually followed by a clause like 'that...' or 'what...'.

What case is the phrase ten szkodliwy nałóg in, and why?

It is in the accusative case because it is the direct object of the verb znać. The addiction is the thing being known.

Why doesn't ten szkodliwy nałóg change its ending in the accusative case?

The word nałóg is a masculine inanimate noun. For masculine inanimate nouns, the accusative case looks exactly the same as the nominative (dictionary) form. Because of this, its modifiers (ten and szkodliwy) also keep their basic dictionary forms here.

Can I put the word dobrze at the end of the sentence?

Yes, Polish word order is flexible. Placing dobrze at the beginning emphasizes how well you know it. Putting it at the end (Znam ten szkodliwy nałóg dobrze) is perfectly correct but slightly shifts the emphasis onto the addiction itself.

Since the current lesson is about verbs like przestać (to stop), can I say Przestałem ten szkodliwy nałóg to mean 'I stopped this harmful addiction'?