Learn Hardcore German: Ungenutzte Räume im Haus sind oft dunkel. - Unused rooms in the house are often dark.

Because it’s the first word of the sentence. In German, adjectives are normally lowercase, but the very first word of any sentence is always capitalized.

Why does Ungenutzte end with -e?

It’s an attributive adjective modifying the plural noun Räume. After no article in the nominative plural you use strong adjective endings; for plural nominative that ending is -e.

What does im stand for, and why is Haus in the dative case?

im is a contraction of in + dem. When in indicates a fixed location (“where?”), German uses the dative case. So dem Haus becomes im Haus.

Why is Haus singular and not plural (Häuser)?

Because the speaker refers to the rooms inside one specific house. If you meant rooms in multiple houses, you’d use in den Häusern.

Why is oft placed before dunkel, and not after?

“Oft” is an adverb of frequency. In a main clause, the typical order is Subject – Verb – Adverb – (other elements) – Adjective. Hence “sind” (verb) comes before “oft” (adverb), which comes before “dunkel” (adjective).

Why is the verb sind in the second position instead of at the end?

German main clauses follow the verb-second (V2) rule: the conjugated verb always occupies the second slot. Here the entire subject phrase “Ungenutzte Räume im Haus” counts as the first slot, so sind comes second.

Can I rephrase the sentence using a relative clause instead of the participle Ungenutzte?

Yes. You could say:
Räume, die im Haus nicht genutzt werden, sind oft dunkel.
This uses a relative clause. The participle adjective version (“Ungenutzte Räume”) is just more concise.

What’s the difference between Räume and Zimmer in this context?

Räume is a general term for any spaces or rooms (storage, attic, cellar, etc.), whereas Zimmer usually refers to living rooms, bedrooms, etc. Here “ungenutzte Räume” can cover all kinds of unused spaces, not just traditional rooms.