A.T. Robertson Commentary 1 Corinthians 14:16

"Else if thou bless with the spirit, how shall he that filleth the place of the unlearned say the Amen at thy giving of thanks, seeing he knoweth not what thou sayest?" — 1 Corinthians 14:16 (ASV)

Else if thou bless with the spirit (επε εαν ευλογηις εν πνευματ). Third class condition. He means that, if one is praying and praising God (10:16) in an ecstatic prayer, the one who does not understand the ecstasy will be at a loss when to say "amen" at the close of the prayer. In the synagogues the Jews used responsive amens at the close of prayers (Nehemiah 5:13; Nehemiah 8:6; 1 Chronicles 16:36; Psalms 106:48).

He that filleth the place of the unlearned (ο αναπληρων τον τοπον του ιδιωτου). Not a special part of the room, but the position of the ιδιωτου (from ιδιος, one's own), common from Herodotus for private person (Acts 4:13), unskilled (2 Corinthians 11:6), uninitiated (unlearned) in the gift of tongues as here and verses 23f.

At thy giving of thanks (επ τη ση ευχαριστια). Just the prayer, not the Eucharist or the Lord's Supper, as is plain from verse 17.