Comentario sobre Pirkei Avot 1:9

AND BE CAREFUL WITH YOUR WORDS, THAT THEY DON’T DISCERN HOW BEST TO LIE THEREFROM. Rav writes, in explaining “they”: the litigants or the witnesses. Although the beginning of the mishna only mentions witnesses, the idea is that they should not discern how best to lie, and that would apply equally to witnesses or litigants. Also, “be careful, etc. ” is an independent sentence unconnected to the beginning of the mishna, and as such the “they” in “that they don’t discern how best to lie” applies to anyone who can so discern, whether witness or litigant.

Shimon ben Shatach says, "Examine the witnesses thoroughly": To examine and cross-examine them many times. And through this you will reveal the secret, since 'with many words sin will not be averted' and he will learn from their words if they are testifying falsely.

"But be careful with your words": That the judge not say, "Maybe the act was like so and so;" or "If the matter had been like so, x would be innocent" - and from these words, the litigant or the witnesses learn to say something that did not [happen].

Introduction This mishnah contains the teaching of Shimon ben Shetach, the second of the third pair. Shimon ben Shetach lived during the rule of Alexander Janneus and his wife Shelomith. Shimon was Shelomith’s brother. According to a legend found in both Josephus and the Babylonian Talmud (with some differences) the king killed many of the Pharisaic sages, but Shimon ben Shetach managed to escape. Through the aid of his sister he was able to return to Jerusalem and rejoin the Sanhedrin, which had been filled in the meanwhile with Sadducees. Along with Judah ben Tabbai, who had escaped to Egypt, they were successful in restoring the Pharisees to their place in the Sanhedrin. That may be why both of these pairs are so concerned with the behavior of judges.

"but be careful with your words, lest from them they learn to lie": As when you examine them about a matter, you can speak your words [in such a way] that they will understand in what way they will be found guilty in their trial and learn from your words what lies they need to make to win [the case].

Shimon ben Shetach used to say: be thorough in the interrogation of witnesses, and be careful with your words, lest from them they learn to lie. Shimon ben Shetach continues to give instructions to the judges. The first statement is self-explanatory. The second statement means that the judge should be careful while examining the witnesses, lest he inadvertently teach them how to help one side win the case.