I am reading through the blue grammar book and the following is the definition of prepositions-
A preposition is a word or set of words that indicates location (in, near, beside, on top of) or some other relationship between a noun or pronoun and other parts of the sentence (about, after, besides, instead of, in accordance with). A preposition isn’t a preposition unless it goes with a related noun or pronoun, called the object of the preposition.
If we consider the sentence- “he is honest”, ‘is’ shows the relationship between ‘he’ and ‘honest’, it tells us he has the quality of honesty, I understand why it is considered a verb but do not understand how it is not a preposition as it satisfies the definition above?
Link for the relevant passage- https://www.grammarbook.com/grammar/probPrep.asp
In many ways “is” functions in a way similar to a preposition . The verb “to be” in its various forms has special functions unlike those of any other verb. None the less, it is a verb. In general English grammar does not use the label preposition for words that form another part of speech, and in particular, not for verbs.
Note that nowhere in the (now) linked article is “is” or any of the other forms of “be” or any other verb listed as a preposition. Prepositions can be used to indicate a relationship, but not usually the relationship of identity or equality “This is that”.
This distinction may not be fully logical, but on the other hand all prepositions normally so defined work as parts of sentences that have verbs other than the preposition itself. If we define “is” in “he is honest” to be a preposition, we would have a verbless sentence. Many verbs define or express a relationship, so the above definition of a preposition cannot be complete and exclusive.