Learn Polish: Rano często jem ser. - I often eat cheese in the morning.

This is exactly how the accusative case works for inanimate masculine nouns! In Polish, non-living masculine objects like ser (cheese) look exactly the same as direct objects as they do in their basic dictionary form.

Where is the word for "I" in this sentence?

It is built right into the verb! Jem means "I eat". Because the verb makes it clear who is doing the action, Polish usually drops the pronoun ja (I) unless you want to add special emphasis.

Most verbs for "I" end in -ę, like piję (I drink). Why is it jem here?

The verb jeść (to eat) is an exception. Instead of the typical -ę ending for the first-person "I" form, it ends in -m, making it jem. You just have to memorize this one!

Why is Rano at the beginning of the sentence?

Polish word order is very flexible! Putting Rano (in the morning) at the start simply emphasizes when the action happens. You could also say Często jem ser rano and it would still be perfectly correct.