Learn Swedish: Pojken äter tre äpplen. - The boy is eating three apples.

In Swedish, the word for "the" usually isn't a separate word before the noun. Instead, it gets attached to the end. Since pojke (boy) is an en-word ending in a vowel, you just add -n to the end to get pojken (the boy).

The translation says "is eating", but isn't äter just "eats"?

It means both! Swedish doesn't have a separate "is doing" (continuous) tense like English does. The present tense verb äter translates to both "eats" and "is eating". You never use a word for "is" combined with an action verb like this in Swedish.

How do you form the plural for a word like äpple?

Äpple is an ett-word (a noun that uses ett for "a" or "one" instead of en). For ett-words that end in a vowel, you form the plural by simply adding -n. So, ett äpple becomes tre äpplen.

Both pojken and äpplen end in -n. Are those endings doing the same thing?

No, they are actually doing two completely different things here! On pojken, the -n means "the" (making it definite). On äpplen, the -n makes the word plural. It is just a tricky coincidence that both grammar rules use the letter -n.