Charles Ellicott Commentary Matthew 21:29

"And he answered and said, I will not: but afterward he repented himself, and went." — Matthew 21:29 (ASV)

I will not—The bold defiance of the answer corresponds to the rough recklessness of the classes (tax collectors and prostitutes) who were represented by the first of the two sons. Their whole life, up to the time of their conversion, had been an open refusal to keep God’s laws and so to work in His vineyard.

He repented—The Greek word is not the same as that of Matthew 3:2 and expresses a regretful change of purpose rather than an entire transformation of character. It is the first stage of repentance and may, as in this instance, pass on into the higher stage, or, as in the case of Judas (Matthew 27:3, where the same word is used), end only in remorse and despair.