"I never thought of it before.

But it don't mean they _must_ have helped somebody, does it?
Seems to me that 'never helped nobody' just naturally fails to say whether or not they helped somebody.

I never thought of it before, and I'll never say it again." She was pleased and surprised with the quickness and surety of his mind.
As soon as he had got the clew he not only understood but corrected her error.
"You'll find it all in the grammar," she went on.

"There's something else I noticed in your speech.

You say 'don't' when you shouldn't.
'Don't' is a contraction and stands for two words.