I do apologise if the question is wordy, but I feel some context is required for me to stand any chance of finding a satifactory answer.
I have been struggling to understand why the word "is" counts as a verb in English. This lead me to search for what verbs actually are rather than the naive definition I was given in school (doing words). I didn't study English beyond highschool, but I am a first language speaker.
I suspect that most speakers would recognise
The sky is blue .
as a grammatically correct clause, but it always seemed to me that the word "is" was simply defined to be a verb in order to force the rule that all clauses must contain a verb. This is really the crux of the question. I found the following definition:
A verb is a word that in syntax conveys an action, occurence, or state of being.
Even on the surface of it this definition must be wrong because several sources tell me that "to be" is a verb despite it being two words and not a word, but so far the gist of it seems to agree with other searches I've done. From what I can tell, an action is a cause for a change in state and occurences are just actions in the past tense so it makes sense to me that they would be lumped them together in a category.
What I don't understand is why states of being are included, since they seem to me to be inherently passive which I understand to be the exact opposite of being active. I've seen the term "state of being verb" used in some explanations, but this only reinforced the idea that a state of being is a separate category to an action.
Have I misunderstood what an action is? Are the definitions I've found just hideously wrong? Or is there something else (besides an education) I'm just missing?