"I like tea."
Why does herbata change it's ending, when mleko for example wouldn't for this sentence. Many thanks
The verb "to like" in Polish needs the noun phrase that is 'liked' to be in Accusative. For neuter nouns, the Accusative form is the same as the Nominative - that is why you have the impression that it doesn't change. But for feminine nouns, Accusative is different and almost always changes the last -a to -ę.
You can read more about Accusative here.
Personal pronouns, especially those for 1st and 2nd person, are generally very redundant in Polish. The form of the verb shows clearly who is the subject. They are redundant to the point of sounding weird, if used. We rather only use them for emphasis.
But they always should be accepted, nevertheless. And 'ja' is accepted here.