Questions & Answers about Ég hjóla heim í seinnipartinn.
Time expressions introduced by a preposition often use the definite form of the noun:
• Only seinniparturinn (the late part) refers to a specific slice of the day.
• The preposition í then governs the accusative (which in this case has the same spelling as the nominative).
Icelandic prefers Place-Time order for adverbials. So you say:
Subject + Verb + Place-adverb + Time-phrase
Eg. Ég hjóla (S-V) heim (place) í seinnipartinn (time).