John 14:16-21 (NASB)
Role of the Spirit
16"I will ask the Father, and He will give you another (A)Helper, that He may be with you forever;
17that is (B)the Spirit of truth, (C)whom the world cannot receive, because it does not see Him or know Him, but you know Him because He abides with you and will be in you.
18"I will not leave you as orphans; (D)I will come to you.
19"(E)After a little while (F)the world will no longer see Me, but you will see Me; (G)because I live, you will live also.
20"(H)In that day you will know that (I)I am in My Father, and you in Me, and I in you.
21"(J)He who has My commandments and keeps them is the one who loves Me; and (K)he who loves Me will be loved by My Father, and I will love him and will (L)disclose Myself to him."
Charles Stanley elaborates on this nicely! :
I am convinced that if Christians truly understood what takes place during prayer, they'd call upon the Lord more often and with greater results. The believer's prayer isn't simply spoken into empty space; the Holy Spirit is there to guide us in offering God-centered requests, fill in the words we cannot say, and carry our needs before the Lord.
The Holy Spirit is part of the Trinity, so He knows the mind of God intimately (1 Cor. 2:11). Since He is as omniscient and omnipotent as the Father, He fully grasps the circumstance we are praying over—even the parts that remain unseen or utterly confusing to us. Also, the Spirit dwells within every believer and knows each one's mind and heart. With this full knowledge, the Holy Spirit can carry out His responsibility to make our petitions match God's desires. Toward that end, He speaks in our spirit and opens our mind to Scripture.
The fact that God gives His Spirit to all believers reveals the value He places on communication between Him and His children. Our Father provides the best possible Helper to ensure that we can be prayer warriors—wise, strong in faith, and fully yielded to Him. But we are none of those things unless we are habitually talking with the Lord.
Christians need never feel guilty for being uncertain about how to pray. The indwelling Holy Spirit knows our needs and desires—as well as the mind of the Father and the details of every situation. He speaks to God on our behalf while teaching us to pray in accordance with His will.
