This is how I explained the er to myself.
er is neither "here" (hier) nor "there" (daar), but it is not "somewhere" or "anywhere" either.
It is a general location, the "er" that is part of every little word that indicates a place even in English (a Germanic language as well), e.g.
here, there, where,
the archaic ones - thither, whither,
and even near (from Middle English nere, ner) and far (Proto-Germanic ferro).
Now, because everything has to happen somewhere and everything is somewhere, the Dutch still use this "basic er" often, whenever the specific location is not important or not known.
The English seem to have decided to drop this "root er", and to make mentions of locations only if they are specific or important to the meaning they are conveying.
it happened somewhere - i do not know where
it could happen anywhere - i do not care where
here - where i am
there - where I can see from where I am
everywhere - all over
However - it happened somewhere but i do not know where nor do i care where and it obviously didn't happen here or there or everywhere - English does not have "a general er" to cover all that. Dutch does - er.
Yes, at the end of a word, you cannot tell the difference between the pronunciation of a -d and a -t.
Among Dutch native speakers, this is actually by far the most common spelling mistake. However, from the grammar rules you can (almost) always tell which one it should be. See here for more info.
Exactly what I was wondering.
It has been explained (above) that if it had "happened over there" it would be "daar" for "there". But that still doesn't explain why you can't drop the "er".
"What has happened?"or "What's happened" is quite a general term in English and doesn't always mean somewhere else. For example you might walk into a room and ask someone this question.