Student or Learner
I've recently heard that when we are referring to the board, we should say "look here" or "look at this" or "look this" but not "look AT here". If so, would you please tell me what's the reason behind of this point?
You need an adverb. Where should you look? Into the distance, at the sky, at this, over there, here, over that tree, into the camera, etc.
"at here" is not grammatical
"this" alone is not an adverb. (at this, into this, though this, etc. would all be okay.)
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.