Learn Icelandic: Seinna vinnur hún ekki. - Later she is not working.

This is the V2 (verb-second) rule in action! Because the sentence starts with the adverb seinna (later), the verb vinnur must take the second slot. This pushes the subject hún to the third spot, right after the verb.

Where does the word ekki (not) go when the subject and verb are flipped like this?

When the V2 rule flips the subject and verb, ekki comes immediately after the subject. In a standard sentence it would be Hún vinnur ekki (She does not work), but because we started with Seinna, it becomes Seinna vinnur hún ekki.

The translation says 'is not working,' but isn't vinnur just the regular present tense for 'works'?

Yes, it is! Icelandic does not have a separate continuous ('-ing') tense like English does. The simple present tense vinnur covers both 'works' and 'is working.' You just use the context of the sentence to know which one sounds better in English.

I learned that 'late' is seint. Why is it seinna in this sentence?

Seinna is the comparative form of seint. Just like in English where 'late' becomes 'later,' the Icelandic word seint changes to seinna to mean 'later.'