Learn Dutch: De berg is hoger dan de heuvel. - The mountain is higher than the hill.

This is because of the Dutch spelling rule for 'open syllables'. In hoog, the double oo is needed to make a long sound because the syllable ends in a consonant. But when you add -er, the word breaks into syllables as ho-ger. The first syllable now ends in a vowel, making it 'open'. A single vowel at the end of a syllable is automatically pronounced long, so writing hooger is incorrect.

Is dan always the word to use for 'than' in comparisons?

Yes, when you use a comparative adjective like hoger, the correct word for 'than' is dan. You will sometimes hear native Dutch speakers say hoger als, but this is considered incorrect grammar. Als is meant for when things are equal, such as in the phrase even hoog als (as high as).

Are berg and heuvel both de words?

Yes, they are both common nouns that take de. This means if you ever place an adjective directly in front of them, like 'the high mountain', you must add the -e ending to the adjective: de hoge berg and de hoge heuvel.

How would I change this sentence to say the mountain is the 'highest'?