We see here “Incline your ear and hear” means humble yourself enough that you will hear. Someone can talk all day, but if you do not receive it, you have done nothing. “Apply your mind to knowledge” is speaking of the knowledge of God being applied to the heart.
Verse 18 is an extension of verse 17. The knowledge of God applied to the heart is a pleasant thing. If you keep it hidden away in your heart forever, your words will be coming from a pure heart and you will be speaking pure words. The knowledge God has revealed to you should not be a shame to you, but a joy to speak.
Proverbs 22:19 So that your trust may be in the Lord, I have taught you today, even you.
We see, we are not to trust in ourselves or even the people around us, but in the Lord who will never let us down. God has revealed Himself to us. This last sentence in verse 19 is saying (believe it or not), I have revealed it unto you.
Proverbs 22:20 Have I not written to your excellent things of counsels and knowledge.
The previous counsels have been the same. He is just reminding them here that they already have been instructed in God’s way.
Proverbs 22:21 To make you know the certainty of the words of truth That you may correctly answer him who sent you?
1 Peter 3:15 But sanctify Christ as Lord in your hearts, always being ready to make a defense to everyone who ask you to give an account for the hope that is in you, yet with gentleness and reverence.
One of the reasons we learn the truth is so that we can pass it on to others after we have applied it to our lives. Not only are we to get saved by the truth of God, but we must minister truth everywhere we go
Under Gods command 