Learn Icelandic: Það pirrar mig þegar lestin er sein. - It annoys me when the train is late.

In Icelandic, hvenær is exclusively used as a question word to ask "at what time?". Because this sentence connects two events and means "at the time that", it requires the conjunction þegar.

Why do we use the accusative mig here instead of the dative mér, which is so common with emotions?

While many Icelandic verbs related to personal feelings (like að finnast or að sýnast) require a dative subject, að pirra (to annoy) works just like a standard action verb. The situation (það) is the subject doing the action, and you (mig) are the direct object receiving it in the accusative case.

Is the word sein an adjective or an adverb here?

It is an adjective describing the train, which is why it takes the feminine nominative form sein to match lestin. If it were an adverb describing how an action was performed (like "he arrived late"), you would use the neuter form seint.

What is the word það doing at the start of the sentence?

Just like the word "it" in the English translation, það acts as a placeholder subject. It gives the verb pirrar a grammatical subject to attach to, representing the general situation of the train being late.