Other answers may be too long or contain a lot of nonsense. What answer will you give to this? Adverbs modify verbs, adjectives, or other adverbs.

Is beer alcohol? Even beer is considered a beverage? This sentence works the same and “usually” obviously modifies “is” since there are no other verbs, adjectives, or adverbs to choose from.

Alan Carmack, you have stated that you have read “highly-regarded linguistic books that state that the verb to be cannot be modified by an adverb. That is ” I would disagree with those books”. If someone asks me “Is the sky usually blue?” and I respond “It usually is,” then “usually” must be modifying “is” since it cannot be modifying “it”. Why can’t the verb “Be” be modified by an adverb?