In English, German, and even Spanish, there is a constituent which appears to be a a gray area between adverbs and conjunctions: the conjunctive adverb, or adverbial conjunction.
Examples of those are:
English: consequently, hence, therefore
German: darum, trotzdem, daher
Spanish: cuando, como, por lo tanto, en consequencia
Oddly, in Portuguese, there seem to be no adverbs classified as conjunctive adverbs. The word groups "por isso" and "em consequência" have exactly the same function as "hence" and "therefore" and bear "por lo tanto" and "en consequencia" as counterparts in Spanish, but they are classified solely as conjunctions, not adverbs.
I would like to know the reason behind this.