"Out" after "to be": adverb, not adjective?

Though out is used predicatively, it acts as an adverb in the sentence.

I came around to see you, but you were out.

We can not say whether a word is an adjective or adverb unless we know its function.

An adjective modifies a noun or pronoun but an adverb modifies an adjective, another adverb or a whole sentence.

You were out means you went out. here out modifies the verb were but not you.because there was some movement or something was not there( it was away)

Here is a link which helps you.

https://www.collinsdictionary.com/dictionary/english/out_1

If the lights are out Here out means not shining which modifies lights so it is treated as an adjective.

If the flowers are out. here out means blossed which modifies flowers so it is treated as an adjective

Here there was no movement of lights are flowers but there was a change in their state.

However , it is really confusing.It can not only be an adjective but also a preposition too.

Here is a link which shows how out can be a preposition too.

https://simple.m.wiktionary.org/wiki/out

I agree that it seems confusing. Out seems like it would be an adjective to describe you. But, it is in fact a place adverb or spatial adverb.

It describes where the action took place. In this case, the confusion is being caused by the fact that your verb is to be (past-tense were). We're conditioned to believe that things that follow that verb are adjectives. But, in this case, what's following it is an adverb of place.

If you substituted another verb the usage becomes clearer:

I came around to see you this morning, but you jumped out.

This works for adverbial phrases, too.

I came around to see you this morning, but you were out in the yard.