Learn Polish: Mam herbatę dla niej.

The basic dictionary form of the word is herbata. However, because it is the direct object of the verb mam (receiving the action of "having"), it needs to be in the accusative case. For feminine nouns that end in -a, you show this by changing the final letter to -ę.

I thought the word for "her" was jej. Why is it niej here?

You are right that the standard word for "her" (when showing possession or using the genitive case) is jej. But Polish has a special rule for pronunciation: when a pronoun comes directly after a preposition like dla (for), an extra n- is added to the beginning. So, jej becomes niej.

Does the preposition dla always change the word that follows it?

Yes! The preposition dla (for) always forces the noun or pronoun that comes after it to take the genitive case. That is why we must use the genitive pronoun form (jej / niej) rather than the basic dictionary form ona (she).

Is it okay to just say Mam instead of Ja mam?