Learn Dutch: Het gras op de heuvel is groener dan hier. - The grass on the hill is greener than here.

You simply add -er to the base adjective. So, groen (green) becomes groener (greener). Because the vowel sound oe is written with two different letters, the spelling doesn't need to change, unlike words with double vowels like groot which drops a vowel to become groter.

Why is dan used here, and not als?

The word dan means 'than' and is the standard word used after a comparative adjective like groener to show a difference. You use als ('as') when comparing things that are equal, like in even groen als (as green as). Note: you might hear native speakers say groener als in casual speech, but groener dan is the grammatically correct written form.

Why doesn't the adjective groener get an -e at the end here?

Adjectives only get an extra -e when they are placed directly before the noun they describe (for example, het groenere gras). Because groener is placed later in the sentence after the verb is, it keeps its base comparative form without the extra -e.

Why do we use the preposition op with de heuvel?