1 Corinthians 14:15

'What is it then? 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.'

So what shall his choice be? This could be interpreted one of two ways. Firstly, that he will choose to pray in both ways. Sometimes to pray in private in tongues so that his spirit is in special rapport with God through the Spirit inspired words, and sometimes to pray in his own language so that as his heart reaches up towards God his mind also understands what he is praying and he can be directly involved rationally. The same then also applies to singing in tongues, and singing with understanding. But if this is in mind, and the context would support it, he has in mind here, at least for the tongues, private prayer, and private singing, for he is aware that the church will not benefit from either. Thus for use in the church he will keep to rational praying and singing.

The alternative meaning is that when he prays in church he will combine spirit and understanding. He wants both to be at work. Then it is referring to the fact that this is what he will do in the church. For he confirms elsewhere that he does use tongues in private prayer.