Learn Swedish: Borsta tänderna på morgonen och på kvällen! - Brush your teeth in the morning and in the evening!

This is a command (the imperative form) telling someone to brush their teeth! For Group 1 verbs like att borsta (to brush), the command form is the exact same as the infinitive form, ending in -a. If it were a normal statement like 'I brush my teeth', you would use the present tense borstar.

Why does the sentence say tänderna (the teeth) instead of 'dina tänder' (your teeth)?

In Swedish, when talking about body parts or clothing, you usually use the definite form (like tänderna) instead of a possessive word like 'your'. Because you are the one being told to brush, it is already obvious whose teeth are being brushed!

How does the word tänderna change from singular to plural?

The singular word is en tand (a tooth). To make it plural, it changes its vowel to become tänder (teeth). Finally, adding the definite plural ending -na gives us tänderna (the teeth).

The lesson is about frequency expressions like om dagen, so why do we use på in på morgonen and på kvällen?