"The ghosts always come at midnight."
Translation:Um Mitternacht kommen immer die Geister.
Something just occurred to me that I didn’t think of when previously encountering this sentence. There’s an ambiguity in the English sentence. It could either mean “On the occasions when the ghosts come, they only do so at midnight”, or it could mean “Every time midnight comes around, the ghosts come.” Does the German sentence also have both meanings, or would, say, a change in word order differentiate the two?
It might help to remember that "am" is a contraction of "an dem", and since "dem" is dative masculine and neuter, it can only precede nouns of those genders. Mitternacht on the other hand is feminine, so you know you can't use "am". (I know it's still a bit confusing as to why you use an Vs um but it might help a little!)
In my mind, Mitternacht is an exact point in time, so you use the same preposition as with times of the day: um 0 Uhr, um Mitternacht. But Mittag is a period rather than a point (I would say roughly from 11am to about 1 or 2pm), so it uses the same preposition as other such periods: am Morgen, am Vormittag, am Mittag, am Nachmittag, am Abend.
Even though I said "midday" for the sake of alliteration in my original post, I've been thinking of "Mittag" as meaning "noon", i.e. 12:00 p.m., so it seemed odd for it to use a different preposition than another word that I also took to be a specific time. Especially when those two words are so closely related (word for "middle" + word for one of the two halves of the day).
I kept the 'm' from "am Mittag" when I typed it above, but it's not the genders that are confusing. It's the use of the two different prepositions.
Um Mitternacht kommen die Geister immer. Why is this incorrect and why does the adverb kick the subject out of its usual position?