Is there a specific reason why, in the NT, the definite article "the" is placed in front of the Holy Ghost in some places but not others in the Greek?
I haven't studied the specifics of the definite article with the Holy Ghost, however, let me offer some generalities about Greek.
In Greek, you typically don't
need the definite article as it is implied in the form of the noun. In fact, when you see the name Jesus in the nominative case it would be proper in Greek to understand it as "The Jesus." But, of course, in English that sounds strange because English typically doesn't work that way.
Certain times, though, the definite article is used to bring out a particular emphasis. It is my assumption, without getting deep into it, that this is why the biblical authors would use the definite article...so that there is no mistake.
So, that's my best guess without getting very technical or diving deep into my books.
Blessings,
The Archangel (see....there's the definite article! The moniker "Archangel" was already taken...so I used the definite article.)