Learn Danish: Fuglen sidder og synger på en ru gren. - The bird is sitting and singing on a rough branch.

Ru means rough and ends in a vowel. Because it describes an indefinite, common gender noun (en gren), it stays in its base form. If it described a neuter noun, it would add a -t (like et rut træ), and in the plural or definite, it would add an -e (den rue gren).

Why does the sentence say sidder og synger instead of just a single verb for is singing?

Danish doesn't have a direct equivalent to the English continuous -ing form. To show that an ongoing action is happening right now, Danish often uses a verb of position (like sidder, står, or ligger) followed by og and the action verb. Here, sidder og synger emphasizes that the bird is currently sitting there while singing.

Are there other adjectives ending in a vowel that follow this same pattern as ru?

Yes, several common adjectives end in a stressed vowel and behave similarly, such as ny (new) and fri (free). They keep their base form for common gender nouns (en ny gren), add a -t for neuter nouns (et nyt træ), and add an -e for definite or plural forms (den nye gren).

Why do we use the preposition på here?

På is the standard preposition for being physically on or on top of something. Just like in English, the bird is resting on the surface of the branch, making på en ru gren the natural translation for on a rough branch.