Learn Icelandic: Ekki sletta á fólkið á kantinum. - Do not splash the people on the edge.

To tell someone not to do something, Icelandic uses ekki followed by the bare infinitive of the verb. You do not use the imperative form for negative commands. If you were telling someone to splash, it would be the imperative slettu, but to say do not splash, you use ekki sletta.

We see the preposition á twice in this sentence, but it is followed by different cases. Why?

The preposition á controls different cases depending on the meaning. When it shows movement or direction, like splashing water onto the people, it takes the accusative case, giving us fólkið. When it shows a fixed location, like standing on the edge, it takes the dative case, giving us kantinum.

What is the base word for kantinum, and how is it formed here?

The base word is kantur, a masculine noun meaning edge, side, or curb. Because it follows á indicating a fixed location, it must be in the dative case. The dative singular form is kanti, and adding the definite article (the -num ending for masculine dative) makes it kantinum.

Shouldn't fólkið change its ending if it is in the accusative case?

The noun fólk is a neuter word. Neuter words always look exactly the same in the nominative (subject) and accusative (object) cases. Adding the neuter definite article gives us fólkið for both, so it has already declined correctly without changing its shape.