In my communication classes. I told them to beware of “you know” because it means the speaker doesn’t know.

“You know” is a speech crutch for the inarticulate..

Points were taken off in speeches where the speaker said “you know” more than once.

Is when “they” say, the American people.

The American people don’t want ugly wind turbines, etc.

And sometimes he'll say, "I know it and you do too". It eliminates any cause for discussion or debate.

Classic propaganda.

Why else would they protect them?

that follows it is true. If everybody knows something, then there is no need to state that fact up front. It's like admitting you are either lying or very unsure of your premise. And, as the OP points out, highly likely that the speaker is desperate to persuade others to believe something that just isn't true.

that everything coming out of a Trumpublican's mouth is a lie, but some refuse to admit it if it furthers their own agenda.

EVERYONE KNOWS!

"these people are Nazis at heart"!!

And if there are some who do not know this, they are missing a "sapiens" from Homo sapiens sapiens!

that someone who uses that phrase is just full of bovine excrement.

And it triggers my old-man Oppositional Defiance brain activity.

"Oppositional defiance, or ODD, is a behavioral disorder characterized by a persistent pattern of anger, irritability, defiance, and vindictive behavior toward authority figures."

I see it as a defense mechanism rather than a 'behavioral disorder'.

No. Everybody does not know that.

Says (you know it's true or I know it and you know it or everybody knows ) to a reporter or show host, that reporter or show host would say shut the fuck up. Or just say, no I don't know that's true. Show me the proof. Or if that's true why haven't you sued? Just something to acknowledge it's a lie and they know it.

Try to point out what the brown stuff in the bowl actually is, and they'll give you a hateful glare and order seconds and thirds, with extra guano sauce, just to savor the appalled look on your face.