Breakdown of Du borde stänga munnen när du tuggar.
du
you (singular)
när
when
stänga
to close
böra
should, ought to
Questions & Answers about Du borde stänga munnen när du tuggar.
Why is there no att (to) before stänga?
Because borde (should) is a modal verb. Just like with kan (can) or vill (want to), modal verbs are followed directly by the bare infinitive form of the next verb. So you say borde stänga instead of borde att stänga.
Why does the sentence use munnen (the mouth) instead of din mun (your mouth)?
In Swedish, when it is obvious whose body part you are talking about, you usually use the definite form (the mouth) instead of a possessive word like din (your). Since the sentence is directed at Du (you), it is clear whose mouth needs to be closed.
Borde ends in -de, which is usually a past tense ending. Is this sentence in the past tense?
No, this sentence is giving present advice. While borde is technically the past tense form of the older verb böra, it is used in modern Swedish exactly like the English word 'should' to give present or future recommendations.
Could I say Du ska stänga munnen instead?
You could, but it would change the tone. Ska makes it a strict command, like 'You will or must close your mouth.' Using borde makes it a piece of advice or a strong recommendation, meaning 'You should close your mouth.'