Word usage from a text
In a song it says: Keiner weiß, was hier geschah
Which makes total sense, it's in the past
Later on in the song it says:
Niemand weiß, was hier geschehen
Could someone tell me why it is geschehen and not geschah
it's a poetic (possibly historic - I'm not sure about that) turn of phrase. It's found in poems but it sounds extremely old-timey. This form of the past tense is never used in spoken german.
It should always be "geschah" or "ist geschehen" in both spoken or written german. You are only allowed to use "geschehen" without the auxiliary verb if you are a 19th century poet - or apparently Rammstein.
PS. why on earth are Rammstein so popular all over the word? I will never understand this.
Almost: the verb "geschehen" needs the auxiliary verb "sein" (to be).
The normal, non-poetic version of this sentence would be "niemand weiss, was hier geschehen ist".
Since its lyrics they are not always following the grammar. So my bets guess is, this is an abbreviation of "was hier geschehen ist". The ist might be removed to suit better into the flow of the song.