Learn Icelandic: Að greina sýrustigið í brennslunni er flókið. - Analyzing the acidity in the roast is complicated.

Because the subject of the sentence is the entire infinitival phrase Að greina sýrustigið í brennslunni (Analyzing the acidity...). When an action or an infinitive clause acts as the syntactic subject, the predicate adjective always takes the default neuter singular form.

Since sýrustigið is also neuter, could flókið just be agreeing with that noun instead?

It looks like a match, but sýrustigið is the direct object of the verb greina (to analyze), not the subject of the main sentence. Even if you analyzed something masculine, like hitann (the heat), the final adjective would still be the neuter flókið because it agrees with the overall action of analyzing, not the specific thing being analyzed.

What dictates the dative case in í brennslunni?

The preposition í governs the dative case when it describes a static location or state, like being physically in the roast or within the roasting process. If there were a sense of movement into a new place or state, í would take the accusative case instead.

How does brennslunni translate to "the roast"?

The feminine noun brennsla literally means "burning" or "combustion" (derived from the verb brenna, to burn). In food and drink contexts, such as evaluating coffee, it is commonly used to refer to the roasting process or the roast itself. Here, it is in its definite dative form.