A.T. Robertson Commentary Matthew 21:16

"and said unto him, Hearest thou what these are saying? And Jesus saith unto them, Yea: did ye never read, Out of the mouth of babes and sucklings thou has perfected praise?" — Matthew 21:16 (ASV)

Hearest thou (ακουεις). In a rage at the desecration of the temple by the shouts of the boys they try to shame Jesus, as responsible for it.

Thou hast perfected (κατηρτισω). The quotation is from Ps 8:3 (LXX text). See 4:21 where the same verb is used for mending nets. Here it is the timeless aorist middle indicative with the perfective use of κατα-. It was a stinging rebuke.