Learn Italian: Hanno studiato un poema antico. - They studied an ancient epic poem.

Even though poema ends in -a, it is a masculine noun. This is because it is a word of Greek origin ending in -ma, just like il problema or il sistema. Since it is masculine, it takes the masculine indefinite article un.

Why does the adjective antico end in -o when the noun poema ends in -a?

Adjectives must agree with the gender and number of the noun they describe. Because poema is a masculine singular noun (despite its -a ending), the adjective antico must take the masculine singular ending -o.

Does poema just mean any type of poem?

Not exactly! A poema usually refers to a long, narrative, or epic poem. If you want to talk about a standard, shorter poem, you would use the feminine noun una poesia.

How do we know who studied the poem if there is no subject pronoun before hanno studiato?

In Italian, the subject pronoun is often dropped because the verb conjugation tells you who is doing the action. The helping verb hanno is the "they" (or loro) form of avere, so it is perfectly clear that "they" are the ones who studied.