Questions & Answers about Het ijs op het water is dun.
Because both ijs and water are het-words in Dutch.
Dutch nouns usually take either de or het as their definite article:
- de for many common-gender nouns
- het for neuter nouns
So:
- het ijs
- het water
The two instances of het are not related to each other here. Each one is simply the article belonging to its own noun.
Because dun comes after the verb is and is being used as a predicate adjective.
In Dutch:
- attributive adjective (before a noun): usually -e
- het dunne ijs
- predicative adjective (after zijn, worden, etc.): usually no -e
- Het ijs is dun
So:
- het dunne ijs = the thin ice
- Het ijs is dun = the ice is thin
This is a very common pattern in Dutch.