Μακάριοι οἱ πτωχοὶ τῷ πνεύματι
Why is the τῷ omitted in English translations? At face value, the proper translations appears to be
Blessed are the poor in the spirit
This is material to me, as I would go further and interpret the spirit to mean the Spirit.
First, @Nigel J made several good points. Wallace's Greek Grammar Beyond the Basics is excellent, and I agree that this is a dative of sphere.
However, this is a case of translation driving hermeneutics. Omitting the changes the range of possible interpretations by excluding the possibility that Jesus meant the Spirit. To be transparent, I believe that is what Jesus was getting at; we are spiritually bankrupt.
Personally, I am guessing that a couple of factors affected the traditional translation. First, Luke 6.20 has Μακάριοι οἱ πτωχοί, which, from the context, is talking about material poverty. Early translators may have been attempting to harmonize Matthew to Luke, a practice with which I personally disagree. Second, translating as the spirit forces a particular conclusion, and the translators may not have been willing to do that.
I asked the question originally to see if anyone knew of where this translation tradition started or if there was some basis in Greek grammar to justify it. I don't think that the use of dative precludes translating the article, but I'm open to being corrected...