Breakdown of Du måste låta bli att bita på naglarna.
du
you (singular)
att
to
måste
to have to, must
på
on / at / for (wait for) / in (believe in)
Questions & Answers about Du måste låta bli att bita på naglarna.
What does låta bli literally mean here?
Literally, låta means "to let" and bli means "to become" or "to be" (similar to the English phrase "let it be"). Together as a fixed phrase, låta bli means "to refrain from", "to stop", or "to avoid doing" something.
Since måste is a modal verb, why is there an att before bita?
You are right that modal verbs take a bare infinitive, which is why låta has no att before it (måste låta bli). However, the next verb in the chain belongs to the phrase låta bli, which specifically requires att before the action you are avoiding. The structure is: modal verb + bare infinitive (måste låta bli) followed by att + infinitive (att bita).
Why do we need the preposition på after bita?
In English you just "bite your nails", but in Swedish, the standard expression is att bita på naglarna (literally "to bite on the nails"). Swedish often uses på for ongoing actions involving parts of the body, such as att suga på tummen (to suck one's thumb).
Why does it say naglarna (the nails) instead of dina naglar (your nails)?