Why does it say "v'Ish... Yehaselu"?
Malbim: Even though one guards himself from his friend [and will not say things that he does not want others to hear], he gossips about him, and says in his name things that he did not say at all.
Why is there a Dagesh in the Lamed in "Yehaselu"?
Radak: This beautifies the pronunciation, due to the stop (the previous letter has a Ta'am Mafsik).
What is the meaning of "Limedu Leshonam"?
Radak: They trained [their tongue to speak Sheker]. This is like "Pereh Limud Midbar" (2:24).
Mesilas Yesharim (Perek 11): Some, it is not their 'trade' to invent stories and events that never happened, but when they come to tell something, they mix in Sheker as they fancy. They became used to this until it became their nature. So is a liar's punishment - even if he says Emes, people do not heed him.
Malbim: They speak total Sheker, without any Emes 1 .
This is difficult. Any Lashon ha'Ra that does not include some truth at the beginning, it will not last (Sotah 35a)! (PF)
What is the meaning of "Ha'ave Nil'u"?
Rashi: They toiled to make their way crooked. Ha'ave is like "v'Hachbed Es Libo" (Shemos 8:11), to make crooked. Radak - Nil'u is like "Te'unim Hel'as" (Yechezkel 24:12).
