In Psalm cxxv, Little Office of the BVM, Baronius Press, I see:
In convertendo Dominus captivitatem Sion: facti sumus sicut consolati:
Which it translates as:
When the Lord turned again the captivity of Sion: we became like men that are comforted.
Using wiktionary.org, I discovered that convertendo is future passive participle ablative. (I tried the Latin dictionary at tuffts.edu: no hits.)
This does not seem to agree with the translation, which speaks of the turning again as if it were a past event.
What is going on here?