What constitutes a lie?

Proverbs 6:16 It is interesting to note, now that I have looked it up
this one also looks to be misunderstood.  But I might talk about that later.

The verse is saying "a tongue that is false".  If the Watchtower of JWs
of July 15, 1981, page 436 is correct, then the Bible would say a soul
or a heart that is speaking falsely, because intention, they say, is involved
in lies that are bad.  I do not believe that.
Everyone will say something false sometime.  It is not sinning to be wrong.
It is sinning to be wrong but be insisting on believing it and detestable to be
teaching it.
That is what they do.  But this is what the Watchtower is saying;
"A liar is one who knowingly utters an untruth designed to deceive and mislead".
Designed to deceive and mislead.
So they who preach can be left in the dark (blind leading the blind)
and because they are left in the dark about the error of what they teach
they are innocent?
For a time they are considered innocent.  But listen to this;
Revelation 2:21
"And I gave her space to repent of her fornication and she repented not"
Do you know what spiritual fornication is?  It does not have to do with sex.