You brood of vipers: see comments on 3.7.
How can you speak good…? may be better expressed as a statement: “That is the way it is with you. You are evil, and so you cannot say good things.” Of course, the original order can be retained, too, with a sentence like “how can you possibly say good things when you are evil?” If the rhetorical question is not natural in the receptor language, then the idea can be expressed as “It is not possible for you to say good, since you are yourselves evil.”
For out of the abundance of the heart the mouth speaks: the noun abundance appears also in the Lukan parallel (6.45) and in Mark 8.8; 2 Corinthians 8.14 (twice). The picture is that of a substance which fills a container to overflowing. Jesus speaks of the heart as though it were a container, which may be filled with either good or evil. And whatever fills it, whether good or evil, overflows through a person’s mouth in the form of the words which he speaks. It should be noted that in Jewish thought the heart does not represent the seat of emotions (as in English), but rather the entire person conceived of as a total being who thinks and wills. New American Bible translates heart as “mind.” It is possible to translate without the metaphor of the mouth, as Barclay indicates: “A man’s words are nothing other than the overflow of what is in his heart.” It may be advisable in other circumstances to do away with the metaphor entirely: “For what a person says reveals what he is really like” or “You can tell what a person is really like by what he says.” Other languages may require the use of other body parts; for example, “Your tongue speaks what is in your liver.”
Many translators have tried to retain the image of the metaphor. One way has been to use a sentence such as “For whatever fills a person’s heart (or, mind), that is what flows out when he speaks” or “For whatever a person is really like, when he speaks, that is the thing that comes out just like a bowl overflowing.”
Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .