In de zomer aan zee.
What do "In de zomer logeer ik hier, een maand of vier." and "Wij zijn met vakantie aan zee." mean?
First sentence: "In the summer, I stay here for about four months." Logeren means staying overnight somewhere. Second sentence: "We are on vacation (or on holiday) by the sea."
In English you can say "a month or two", but I don't think you can say "a month or four", like you do in Dutch. Language keeps amazing me.
It depends. A general "We are on vacation/on holiday" would be said as "We zijn op vakantie" or "We zijn met vakantie." Both are correct Dutch. The "met vakantie' phrase can also mean that you're off from work, but not necessarily going 'on vacation' somewhere, just staying at home. Which preposition you use after 'op vakantie/met vakantie' depends on the location. You'd use 'aan' if it's by the sea (aan zee) or at the coast (aan de kust), but you'd use 'naar' if going to a different country, "We zijn op vakantie naar Spanje." You could even use 'bij', "We zijn op vakantie bij mijn familie in New York."
I don't really know. It's just one of those Dutch oddities. It's the same with "aan tafel" e.g. We zitten aan tafel.