Jamieson, Fausset and Brown on 1 Corinthians 14:15

15. What is it then?--What is my determination thereupon?

and--rather as Greek, "but"; I will not only pray with my spirit, which (1Co 14:14) might leave the understanding unedified, BUT with the understanding also [Alford and Ellicott].

pray with the understanding also--and, by inference, I will keep silence altogether if I cannot pray with the understanding (so as to make myself understood by others). A prescient warning, mutatis mutandis, against the Roman and Greek practice of keeping liturgies in dead languages, which long since have become unintelligible to the masses; though their forefathers spoke them at a time when those liturgies were framed for general use.