Questions & Answers about Drengen har bygget slottet med vådt sand.
Why does the adjective take the form vådt instead of just våd?
Because sand is an uncountable mass noun of the neuter gender (et sand). When an adjective comes before an indefinite neuter noun, it must agree by adding a -t. So våd becomes vådt.
Do adjectives for all mass nouns get a -t added to them?
No, it completely depends on the gender of the noun. You only add the -t for neuter mass nouns like sand or vand. If you were talking about a common gender mass noun like sne (snow), the adjective would stay in its base form without a t: våd sne (wet snow).
Why is it slottet with a double t?
The base word is et slot (a castle), which has a short vowel sound followed by a single consonant. When you add the definite ending -et to say the castle, that final consonant doubles to keep the vowel sound short, making it slottet.
Why use the present perfect har bygget (has built) here instead of the simple past byggede (built)?
Just like in English, using the present perfect connects the past action to the present. Saying the boy har bygget the castle emphasizes the physical result that exists right now, whereas byggede would just state what he did at some specific time in the past without tying it to the present moment.