Learn Swedish: Pojken glömmer sitt paraply. - The boy forgets his own umbrella.

Because sitt means "his own". When the third-person subject of the sentence (the boy) is also the owner of the object, Swedish uses the reflexive pronouns sin, sitt, or sina. If the sentence were Pojken glömmer hans paraply, it would mean the boy is forgetting some other boy's umbrella.

Why is it sitt and not sin or sina?

The choice between sin, sitt, and sina depends on the thing being owned, not the owner. Because paraply (umbrella) is an ett-word (ett paraply), we must use the ett-word form, which is sitt. If he forgot an en-word like a coat (en jacka), it would be sin jacka. If he forgot plural items, it would be sina.

Since it is his specific umbrella, why isn't paraply in the definite form?

In Swedish, you never use the definite form of a noun immediately after a possessive word (like min, din, hans, or sitt). The possessive word already makes it specific enough, so the noun stays in its basic, indefinite form.

How does the word pojken mean "the boy" instead of "a boy"?