Learn Icelandic: Hvenær fer Jón til Maríu? - When is Jon going to Maria's?

The preposition til (to) requires the noun that follows it to take the genitive case. For feminine names that end in -a, like María, the genitive case ending is -u.

Does til Maríu mean he is going to her as a person, or to her house?

When you use til followed by a person's name, it usually implies going to their home, room, or place of work. It translates perfectly to the English phrase "to Maria's".

The English translation says "is going", but isn't fer just the simple present tense?

Yes, but Icelandic does not have a separate present continuous tense like English does. The simple present tense fer (from the verb fara) can mean "goes", "is going", or even "will go" depending on the context of the sentence.

Why does the verb fer come before the subject Jón?

Icelandic strictly follows the "verb-second" rule. Because the sentence starts with the question word Hvenær (when), the verb fer must take the second slot, bumping the subject Jón to the third slot.