Breakdown of Mam bliźniaka, który ma taki sam charakter jak ja.
ja
I
mieć
to have
który
which, who (relative)
bliźniak
twin (m)
Questions & Answers about Mam bliźniaka, który ma taki sam charakter jak ja.
Why is it taki sam and not taka sama or takie same?
It has to agree in gender, number, and case with the noun it modifies, which is charakter. Since charakter is a masculine inanimate noun and is in the accusative case here (as the object of ma), we use the masculine inanimate form taki sam.
Why is it bliźniaka with an -a at the end?
Bliźniak (twin) is a masculine animate noun. Because it is the direct object of the verb mam (I have), it takes the accusative case. For masculine animate nouns, the accusative case looks exactly like the genitive, which adds an -a.
Why does the pronoun który (who) not match the accusative case of bliźniaka?
Który is the subject of its own part of the sentence (który ma... meaning "who has..."), so it must be in the nominative case. Relative pronouns take their gender and number from the noun they refer to (masculine singular), but their case comes from their role in their own clause.
Why is it jak ja at the end instead of an object pronoun like mnie?
In Polish comparisons with jak, the pronoun matches its role in the implied full sentence. Here, the hidden thought is "as I have" (jak ja mam). Since ja is the subject of that implied action, it stays in the nominative case.