The word forms doch, aber, denn, jedoch can be
used as coordinating conjunction:
er war wütend, denn er hatte die Zahnpasta vergessen.
but also purely adverbial:
wo hatte er sie denn liegengelassen?
In a varity of cases, there is a reading between adverb and
conjunction, but because coordinating conjunctions don't change the
position of the verb in German, the position of the verb cannot be
takene as a test.
Tagging practice STTS/IMS-TUE:
doch, aber, denn, jedoch
are tagged as KON when they appear as first word of a sentence
or as first word after a comma in a complete coordinated sentence. In
all other situations, they are tagged as ADV.