Learn Swedish: Tränar hon oftare än du? - Does she train more often than you?

The basic word for 'often' is ofta. To say 'more often' (the comparative form), you add -re to the end, giving you oftare. Regular adverbs that already end in a vowel usually just take -re.

Why does it say än du instead of än dig?

In Swedish, än (than) links to a hidden part of the sentence. The full idea here is 'than you train' (än du tränar). Since 'you' is the subject of that hidden verb, we use the subject pronoun du. English usually uses the object pronoun ('than me' or 'than him'), so this is a common trap!

Why is there no word for 'does' in this question?

Swedish doesn't use helper verbs like 'do' or 'does' to ask questions. Instead, you just put the verb first. So 'She trains' is Hon tränar, and 'Does she train?' becomes Tränar hon? (literally 'Trains she?').

If the sentence was about multiple people ('Do they train more often...'), would oftare change form?

No, it stays exactly the same. Oftare is an adverb because it describes the action (the verb tränar). Unlike adjectives, adverbs never change their endings to match plural subjects.