Ex- In the sentence "look here" here is the adverb of the verb. If I write "look at here" why is this wrong? Can't I consider "at here" as the adverbial phrase with at being the preposition and here being used in noun sense. How to know when an adverb can be used and when an adverbial phrase can be used?
"Look here" is a common phrase so people understand "here" in this context to be an adverb modifying "look." At is a preposition that requires an object. When you make the object of "look" be "here," people are expecting here to function as an adverb instead, so it sounds unnatural. "Here" is an adverb that does not require a preposition, so why would we add the preposition "at?" When referring to a concrete noun ("look at this") then use at. When referring to a general thing, do not use at ("look over here").
In some really weird contexts, it could actually be okay to say "look at over here," but I would not try this. "Look at here" is hard to every justify even in the weirdest of contexts.