Learn Icelandic: Ekki skola mölina með sápu. - Do not rinse the gravel with soap.

In Icelandic, negative commands are almost always formed using ekki followed directly by the infinitive. While you could technically say skolaðu ekki, using ekki plus the infinitive is much more natural and is the standard way to tell someone not to do something.

The preposition með can take either the accusative or the dative case. Why is it followed by the dative sápu here?

When með indicates the instrument, material, or means used to perform an action (in this case, washing by means of soap), it always takes the dative case. It generally only takes the accusative when it implies "bringing" or "carrying" something along with you.

What kind of noun is möl, and why does the definite article look like -ina here?

Möl (gravel) is a strong feminine noun. Because it is the direct object of skola, it is in the accusative case. For this specific noun, the accusative form is identical to the nominative (möl). When we attach the feminine accusative definite article -ina, we get mölina.