For the people shall dwell in Zion at Jerusalem - He will regather people out of their captivity.
You shall weep no more - Jeremiah tells us they come back weeping (Jeremiah 31:9), but they will soon have cause to cease their crying.
He will be very gracious unto you at the voice of your cry - In Nehemiah, when the people cried out to Him, He rescued them. He sent deliverers, saviors, to them.
When He shall hear it, He will answer you - How did He answer Israel's prayers historically? He answered by sending a Moses or a Joshua. He will follow the same pattern during the Millennium.
Yet shall not your teachers be removed into a corner any more - The "Moses" or the "Joshua" will not die and be replaced by somebody inferior.
But your eyes shall see your teachers - This is an expression of longevity, as their teachers will be eternal.
John W. Ritenbaugh
Preparing to Be a Priest
Other Forerunner Commentary entries containing Isaiah 30:21: