"For if I pray in a tongue, my spirit prays, but my mind is unfruitful." (1 Corinthians 14:14)
Paul said that when we pray in tongues, it's our spirits which are praying (through the Holy Spirit), not our minds (because our minds are unfruitful while we pray with our spirits, Paul said). I see "praying in tongues" and "praying with my spirit" and "praying in the Spirit" all mean the same thing.
First, you are jerking this text out of its context. Paul's argument is very very clear. There is
no profit in speaking in tongues if no one can understand what is being said. That is the whole point of verses 6-12
6 ¶ Now, brethren, if I come unto you speaking with tongues, what shall I profit you, except I shall speak to you either by revelation, or by knowledge, or by prophesying, or by doctrine?
7 And even things without life giving sound, whether pipe or harp, except they give a distinction in the sounds, how shall it be known what is piped or harped?
8 For if the trumpet give an uncertain sound, who shall prepare himself to the battle?
9 So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.
10 There are, it may be, so many kinds of voices in the world, and none of them is without signification.
11 Therefore if I know not the meaning of the voice, I shall be unto him that speaketh a barbarian, and he that speaketh shall be a barbarian unto me.
12 Even so ye, forasmuch as ye are zealous of spiritual gifts, seek that ye may excel to the edifying of the church.
13 Wherefore let him that speaketh in an unknown tongue pray that he may interpret.
In the context where tongues is being used inappropriately (in the church, to believers, not to the jews) this person is violating the principle of love and thus speaking "mysteries" or "speak into the air" things incomprehensible to all present in that context and no one profits from it. Only God knows what is being spoken. He simply is BUILDING UP himself by a FALSE APPEARANCE of spirituality. Paul's conclusion is exercise spiritual gifts for edifying.
Paul follows this reasoned principle (vv. 6-11) and conclusion (vv. 12-13) with his own personal illustration meant to reinforce his conclusion in verses 12-13 and show what he means. He first introduces "IF" considerations followed by "I WILL" determinations.
14 For if I pray in an unknown tongue, my spirit prayeth, but my understanding is unfruitful.
This is precisely what he previously described as to "speak into the air" and what he previously denied provided any "profit." The sounds are being emitted for serve not fruitful purpose either to the listeners or the speaker. This is what he previously described as sounds without "distinction" and "barbarian" in nature. The activity of his spirit serves no edifying purpose for anyone.
15 ¶ What is it then? He has already answered that question! It is "unfruitful" worthless, empty sounds in the air, barbaric, nothing other than drawing attention to self and thus edifying in a negative sense - building yourself up before people by an appearance of spirituality.
I will pray with the spirit, and I will pray with the understanding also: I will sing with the spirit, and I will sing with the understanding also.
Notice everything said previously is introduced by the "IF" but here there is an assertion of what Paul will do in contrast to what he will not do but only considered by "IF".
Note the words "I will PRAY WITH the spirit" showing that prayer with comprehension and understanding can be joined "with the spirit" and thus "in the spirit" includes praying with comprehension as much as without comprehension and so he is not teaching that to pray "in the spirit" means only by tongues. He could have added any other variety of expressing than singing. He could have said, preaching, teaching, exhorting, etc. There are not different kinds of tongues but tongues is some known language that can be verbalized any number of ways (preaching, teaching, prayering, singing, etc.). It all comes out of the mouth.
16 Else when thou shalt bless with the spirit, how shall he that occupieth the room of the unlearned say Amen at thy giving of thanks, seeing he understandeth not what thou sayest?
17 For thou verily givest thanks well, but the other is not edified.[/I]
He has stated what he will not do versus what he will do. He will use tongues "with understanding" but that too does not meet Paul's rule of edification if there are others listening because they do not understand what you are saying.
Your problem is that you jerk words, phrases and verses out of context and then form your own context to interpret them. That is simply being dishonest with God's Word.