Consider these examples:
They are everywhere. There is food everywhere.
I used sentences like these a lot but lately I realize that everywhere is an adverb. What about those grammar rules that say "using adjective with linking verb" ?
Some “adverbials” can act as adjectives. Locatives in particular—expressions which designate a location in time or space—can modify a nominal as well as an “action”, and thus may be properly employed as the complement of a linking verb:
Traditional grammarians sometimes try to get around this awkward fact by claiming that these “adverbials” modify the linking verb; but this is clearly wrong, since a) this leaves the verb without a complement, and b) the same constituents can be employed in contexts where they clearly modify nominals:
An adverb modifies a verb or an adjective.
You are correct that in sentences with a linking verb, the predicate modifier is usually an adjective. That is because the modifier is refering to the subject, which is a noun or pronoun.
However, in a few senteces using verb that may be considered linking, the predicate modifier is actually modifying the verb. In the exaples you give, everywhere modifies is and are.
The rule "After to be follows an adjective" is simply formulated wrong.
If you read texts carefully you will see that the rule "After to be follows an adjectve" is simply wrong.
You have to reformulate your rule: one of the sentence types with to be is to be + adjective. But there are also sentence types such as to be + noun or + adverbials/adverbs.
When you ask when was it the answers can be - That was after the war/ in1963/ on last Monday/recently/yesterday. Here you have when-indications after to be, and no adjectives.
If you clarify the question, it'll be better. What you want to know? If you firmly believe that an adverb should not be used after a linking verb, I'm afraid, this is incorrect.
I'm not aware of the rule that denies a linking verb not taking an adverb after it. The simplest example is...
Yes, we have rules for adverbial positioning and I think it'll be helpful.
The adverbs of manner (how), place (where -this is your case), and time (when) generally go in end position.
Another thing, Swan's book describes that in such cases, here and there begins the sentences. So again,
It rocks! When we generally go by rules, we have to accept some exceptions to them. Using adverbial complements for linking verbs is yet another example for this exception. Normally it goes by written rules. But, when we ask question like 'where?', we have no other option than giving an adverb as answer.
For example, if anyone asks, 'Where is she?', you cannot answer, 'She is pretty' or 'She is a student'. Here, you have to tell where she is. To express her location, you have to use an adverb or adverbials (as we all know, adverbs only can be the answer for such questions). Hence, the answer could be 'She is in New Delhi' or 'She is on the upstairs' or ' She is in the next room' and so on.
I hope it's clear.