Learn Polish: Dwie krowy jedzą smaczne jabłka. - Two cows are eating tasty apples.

The word krowa (cow) is a feminine noun. In Polish, the number 2 has different forms depending on gender. You use dwie for feminine nouns, whereas dwa is used for neuter or masculine animal/inanimate nouns. Because the number is 2 (and the rule is the same for 3 and 4), it is followed by the basic nominative plural form, which is krowy.

Why does the plural word jabłka (apples) end in -a, making it look like a feminine singular noun?

The singular word is jabłko, which is a neuter noun ending in -o. To make neuter nouns plural, the -o changes to -a. In this sentence, the apples are the direct object being eaten, so they are in the accusative case. Luckily, for all plural nouns except "masculine personal" ones (groups including men), the accusative plural looks exactly the same as the nominative plural!

Why does the adjective smaczne end in -e?

Adjectives must match their noun's gender, number, and case. Because jabłka is a plural, non-masculine-personal noun in the accusative case, the adjective describing it takes the -e ending. (Note that the singular neuter form smaczne jabłko also ends in -e, but here it is specifically the plural ending).

I don't recognize the verb ending for jedzą (are eating). What is the infinitive?