The first clause is in Matthew only. O generation (ye offspring, Revised Version) of vipers (Matthew 3:7, note). Observe that the figure of the tree had also been used by the Baptist (Matthew 3:10). How can ye.
It is against nature. Being evil; i.e. inherently worthless (Matthew 6:13, note); cf. πονηροὶ ὄντες, Matthew 7:11. Speak good things. For out of the abundance; i.e. even to overflowing. Of the heart the mouth speaketh.
In Ephesians 4:29 there is apparently a reminiscence of this saying in connexion with our verse 33 (cf. also James 3:10-12).