Learn Swedish: Pojken äter den tredje kakan. - The boy is eating the third cake.

In Swedish, when you use a describing word (like the number tredje) before a definite noun, you have to use what is called "double definiteness." This means you use the separate word for "the" (den) AND add the definite ending to the noun (kakan).

How do I know whether to use "den" or "det" before "tredje kakan"?

You look at the noun it belongs to! The word for cake is an en-word (en kaka), so you use den. If it were an ett-word, like ett äpple (an apple), you would use det tredje äpplet.

Does "äter" mean "eats" or "is eating"?

It means both! Swedish doesn't have a separate verb form for things happening right now (like the English "is eating"). You use the present tense äter for both "the boy eats" and "the boy is eating".

Why is it "pojken" and not "pojke"?

Pojke means "boy", but the English sentence says "the boy". To say "the" in Swedish when there is no adjective, you attach the ending to the end of the noun. For an en-word ending in a vowel, you just add -n, making it pojken.