If you find yourself debating whether to use “was” or “were” in a sentence, it’s likely that you’re dealing with an unreal conditional sentence. As a refresher, an unreal conditional sentence expresses events that are hypothetical or improbable.
The phrase "never met him yesterday" is incorrect because "never" (implies at no time in life) and "yesterday" (refers to a specific past time) should not be used together.
Correct sentence: "You are wrong; I met him yesterday." or "You are wrong; I have never met him." (without "yesterday").