Learn Swedish: Pojken klär på sig. - The boy is getting dressed.

sig is a reflexive pronoun. In Swedish, you use it when a third person (like he, she, it, or they) does something to themselves. Literally, this sentence translates to The boy dresses on himself. You must use sig here because he is the one receiving his own action.

Why is på included here?

klär på sig works together as a set phrase meaning to get dressed or to put clothes on. While klär sig generally means dresses (as in, he dresses well), adding på focuses on the physical action of putting clothes on your body.

The English translation says is getting dressed. Where is the is in Swedish?

Swedish doesn't use a separate word for is when talking about ongoing actions like English does with its -ing words. The single present tense verb klär translates to both gets dressed and is getting dressed. You just choose the one that makes the most sense in English.

How does pojken mean the boy?

In Swedish, the word for the isn't a separate word before the noun. Instead, it attaches to the end. pojke means a boy, and adding -n to the end makes it pojken, meaning the boy.