Falsehood is bad in the eyes of both God and Man, for a liar is despised and repulsive. It is obviously shameful. Those who speak falsely in their business dealings with non-Jews cause a desecration of Hashem’s Name — and the severity of desecrating Hashem’s Name is well-known. Besides this sin, falsehood leads to several other prohibitions as well. This, too, is obvious. Ultimately, the liar receives his punishment: Even when he tells the truth, no-one believes him.
The Midrash tells of a man who was thoroughly sinful, but when he accepted upon himself to speak only the truth, he turned around completely and repented. Each time he was about to commit a sin, he thought: “If anyone asks me about what I’ve done, what will I answer? — if I tell the truth, I will be embarrassed and shamed, but if I lie, I will be breaking my commitment.” In this way he was able to control himself and so he did not “walk in the counsel of the wicked nor stand in the path of the sinful” (Tehillim 1:1).
Seeing how great is the virtue of truthfulness and how disgraceful is falsehood, it is proper to be strong and train one’s children in the trait of truthfulness. The children should see this first-hand in the parent, when they see how careful the parent is to speak only the truth, then they will do the same. A parent should thus vigilantly watch over his children so that nothing dishonest be found in their mouth, and if they do something improper, he should lead them to admit the truth. And when they do so admit, he should forgive them and not punish them, and instead he should just encourage them to never do that thing again. On the other hand, if they lie about what they’ve done, then they should be punished for the falsehood as well. Falsehood should become such a grave matter to them that it becomes part of their nature to never tell a lie.