Usage of since: He hasn't met her since last christmas...

[Sosashi]

"He hasn't met her since last christmas."
"He was the most greatest inventor since Edison"

I'm wondering if he met her last christmas.
I'm wondering if he was a greater inventor than Edison.

please let me realize your teaching.

[Expatcat]

"He hasn't met her since last christmas." In this sentence "since" is an adverb meaning from then till now. Then= last Christmas

So: He met her last Christmas. He has not met her again between the last meeting on Christmas and the current time.

"He was the most greatest inventor since Edison" (the word most does not belong in the sentence) In this sentence "since" is a conjunction meaning in the period of time after the time of Edison.

So: In the period of time following Edison's time, he was the greatest inventor.

"I'm wondering if he was a greater inventor than Edison."

The sentence does not say if he was greater or not, only that he was the greatest inventor in a period of time following Edison's time.