The fact that singing is mentioned together with prayer (see also 1 Corinthians 14:26) suggests that, in the New Testament church, prayers were often sung. Perhaps the gift of tongues was at times also exercised in the form of singing.
When one prays or sings praises to God, the mind
(understanding) must be fully engaged. Paul uses the future tense to express determination. He will certainly pray and sing with both his spirit and his mind.
15 What am I to do? I will pray with my spirit, but I will pray with my mind also; I will sing praise with my spirit, but I will sing with my mind also.