Learn Polish: Wczoraj zapłaciłem za roczną subskrypcję. - Yesterday I paid for the annual subscription.

When talking about paying for something (a transaction), the verb zapłacić takes the preposition za. In these transactional phrases, za is always followed by the accusative case, which is why we see roczną subskrypcję instead of the dictionary form roczna subskrypcja.

Both roczną and subskrypcję are in the accusative case, so why do their endings look different?

In the accusative singular, feminine adjectives take the ending -ą, which is why we have roczną. Feminine nouns ending in -a (like subskrypcja) change that to -ę, giving us subskrypcję. This mismatch between -ą and -ę is completely normal for feminine adjective-noun pairs in this case.

Could I say płaciłem instead of zapłaciłem?

Since the sentence says wczoraj (yesterday) and describes a single, successfully completed payment, you need the perfective verb zapłaciłem. If you used the imperfective płaciłem, it would mean "I was paying" (an ongoing or interrupted action) or "I used to pay" (a repeated habit).

Does the verb zapłaciłem tell us the gender of the speaker?