Learn Icelandic: Það er stundum erfitt að æfa framburðinn. - It is sometimes difficult to practice the pronunciation.

Because it agrees with the dummy subject það (it). In this extraposition sentence structure, það holds the place of the actual subject (the action of practicing), and since það is neuter singular, the adjective describing it must be neuter singular too.

Why does the adverb stundum come before erfitt?

In Icelandic main clauses, adverbs like stundum (sometimes) usually directly follow the finite verb. Since the verb er (is) must be in the second position due to the V2 rule, the adverb slots in right after it, before the adjective.

Why is framburðinn in the accusative case here?

It is the direct object of the verb æfa (to practice), which governs the accusative case. The dictionary form is the masculine noun framburður (pronunciation). In the accusative, it drops the -ur to become framburð, and adding the accusative definite article makes it framburðinn (the pronunciation).

Can I drop the að before æfa?

No, you need að here. When an infinitive verb acts as the delayed subject of a sentence (explaining what it is difficult to do), it requires the infinitive marker að (to) to link it properly to the rest of the sentence.