Learn Swedish: Du behöver inte skölja munnen igen. - You do not need to rinse your mouth again.

You drop the final -a from the infinitive verb and add -ning. This turns skölja into en sköljning (a rinse or a rinsing). Remember that verbal nouns ending in -ning are almost always "en" words.

The English translation says "your mouth", but the Swedish sentence uses munnen (the mouth). Why?

In Swedish, when it is obvious from the context whose body part is being talked about, you use the definite form (like munnen) instead of a possessive word like din (your). Since you are the one rinsing, it is automatically assumed to be your own mouth.

Why isn't there an att (to) before skölja?

Even though we say "need to" in English, the Swedish verb behöva (to need) often acts like a modal verb (such as "can" or "must"). When it is followed directly by another verb, you drop the att and simply write behöver skölja.

How do you pronounce the sk at the beginning of skölja?

It is pronounced with the Swedish "sj" sound, which sounds a bit like a deep English "sh" or a breathy "hw" depending on the dialect. This happens because ö is a "soft" vowel, which softens the sk combination that comes right before it.