why I don't say, "Seit einem paar Tagen laufen Markstudien.
"ein" and "paar" belong together and "ein paar" is unchangeable. That phrase is a little special: You use it like an adjective but must not decline.
In this sentence "Tagen" is indefinite plural and has no article. "ein" and "Tagen" don't belong together or would you say "one days" in English?
You can cut out "ein paar" and keep the same grammatical basic structure but you get a different meaning: "
Seit (ein paar)
Tagen laufen Marktstudien." ("seit Tagen" = for days) I think that's very similar to English, isn't it?
@Salivia
I had to learn this mnemonic rhyme in school: "mit, nach, von, seit, aus, zu, bei
fordern stets Fall Nummer Drei"