Learn Icelandic: Við stöndum hjá háum fossi.

The preposition hjá (by/next to) requires the words following it to be in the dative case. Because we are describing a masculine noun (foss) without the word "the", the adjective takes the strong masculine dative singular ending -um, becoming háum.

How do we know to use the strong adjective ending instead of the weak one here?

In Icelandic, adjectives use the strong declension when they describe an indefinite noun—meaning there is no definite article (no attached "the") or pointing word like "this" or "that". Since we are saying "a high waterfall" rather than "the high waterfall", we use the strong form.

Why is the word for waterfall fossi instead of just foss?

The dictionary form is foss, which is a masculine noun. Just like the adjective, the noun itself must change to match the dative case triggered by the preposition hjá. For many masculine nouns, the dative singular is formed by adding an -i to the end, making it fossi.

I know the verb for "to stand" is standa. Why does it have an ö in stöndum?