Learn Swedish: Vi börjar att städa huset nu. - We are starting to clean the house now.

Att is the Swedish infinitive marker, working just like the word to in English. When you connect two verbs, like starting to clean (börjar att städa), you use att before the second verb to show it is the basic dictionary form of the action.

Why does städa end in an -a? Shouldn't present tense verbs end in -r?

The first verb, börjar, is in the present tense and correctly ends in an -r. However, städa is not in the present tense here; it is in the infinitive form because it comes after att. In Swedish, almost all verbs end in an -a in this basic infinitive form.

The English translation uses are starting, but doesn't börjar just mean start?

Swedish doesn't have an equivalent to the English -ing verbs (the continuous tense). Because of this, börjar covers both start and are starting. You just pick whichever sounds best in English based on the context, like the word nu (now) in this sentence.

Why is the word for house written as huset instead of just hus?

Hus simply means house, but this sentence is specifically about the house. Since hus is an ett word (ett hus), you attach -et to the end of it to make it definite, giving you huset.