Learn Icelandic: Að velja góða brennslu er erfitt. - Choosing a good roast is difficult.

The adjective erfitt does not describe the feminine noun brennslu. Instead, it describes the entire action of the infinitive clause: að velja góða brennslu. Whenever an entire phrase or infinitive clause acts as the subject of a sentence, Icelandic treats it as a neuter singular concept. Therefore, the adjective takes the neuter singular form erfitt rather than the feminine erfið.

Even though the phrase is the overall subject, why are góða and brennslu in the accusative case?

They act as the direct object of the infinitive verb velja (to choose). Even when an infinitive verb is part of the subject clause, it still dictates the case of its own objects. Because velja governs the accusative case, the feminine noun brennsla becomes brennslu, and its adjective matches it as góða.

Is it more natural to start the sentence with það instead of the infinitive phrase?

Yes, it is often more natural in everyday speech to use a dummy pronoun and say Það er erfitt að velja góða brennslu. Putting the infinitive clause at the very beginning, as in this sentence, is grammatically flawless but sounds a bit more formal or emphasizes the action of choosing itself.

Does the word brennsla only refer to a roast in a food or coffee context?