Learn Swedish: Du blir varm allteftersom du skrubbar dig på ryggen. - You get warm gradually as you scrub your back.

In Swedish, when you perform an action on your own body, you usually do not use a possessive pronoun like din (your). Instead, you use a reflexive structure: the verb (skrubbar) + a reflexive pronoun (dig) + a preposition (på) + the body part in its definite form (ryggen). It literally translates to 'scrub yourself on the back.'

How do I know which preposition to use? Why is it på ryggen rather than i ryggen or om ryggen?

The choice of preposition depends on the body part and the action. På is typically used for broad surfaces or when the action happens on the outside of a body part, like the back, stomach, or cheek. You would use i for something penetrating, indenting, or grabbing (like bita sig i läppen - bite one's lip), and om for areas you wrap around or symmetrical pairs (like tvätta sig om händerna - wash one's hands).

Is allteftersom a single word, and what exactly does it mean here?

Yes, allteftersom is a single conjunction. It means 'gradually as' or 'in proportion as.' It emphasizes a parallel process: the action of getting warm is happening simultaneously and at the same rate as the action of scrubbing.

If I wanted to add inte (not) to the second part of the sentence, where would it go?