Learn Polish: Lubię popołudnie. - I like the afternoon.

In Polish, the verb ending tells you who is doing the action. The -ę at the end of lubię already means "I like," so you don't need to add the pronoun ja (I) unless you want to emphasize it.

I learned that "lubię" requires the accusative case. Why didn't the ending of popołudnie change?

You are right that the verb lubić (to like) makes the noun follow it take the accusative case. However, popołudnie is a neuter noun because it ends in -e. In Polish, neuter nouns look exactly the same in the accusative case as they do in their basic dictionary form.

The word popołudnie looks very long. Is it made up of smaller words?

Yes, it is a compound word that literally means "afternoon"! It is made from the preposition po (after) and the noun południe (noon).

What if I want to say "I love the afternoon"? Can I use kocham instead?

You could, but in Polish, the verb kochać (to love) is a very strong word usually saved for people you love deeply, like family. For everyday things, times of day, or hobbies, it sounds much more natural to say bardzo lubię (I like a lot / I really like).