Here is how I see it. For a word to be an adjective, it must be describing some object. If you say, “The pot was watched”, watched is not a property of the pot. There is an irony in “A watched pot never boils”. The sentence goes against our perception that being watched has no effect on the pot and should have nothing to do with whether or not it boils.

In the last example, I can see “tarnished” used as either an adjective or passive voice, although the more common use would be as an adjective. The focus would ordinarily be on the current state of the pot rather than acts of tarnishing that had been made on it.

Part of the issue is the way that we are primed in school to see parts of speech as strict categories whose lines never blur. The fact is that a participle is a verbal adjective and has properties of both a verb and an adjective. Many participles become full-fledged adjectives independent of a verbal construction. No one would try and say that the word “broken” in the phrase “the broken glass” is a verb. Its origins are verbal but in that phrase it’s an attributive adjective describing “glass”. Adjectives describe the state of something.

Passive construction is fundamentally a combination of the helping verb “to be” and a verbal adjective (a participle). There is a fine line between analyzing a phrase as passive voice or as linking verb + adjective. One way to differentiate is that passive voice should either have a subject and agent or an implied agent. In the sentence “the pot was tarnished before I polished it”, you’re referring to the state of the pot, not a recent action performed by some agent. Cf. the difference between “the windows in the house are broken” and “the windows in the house are broken regularly”. The presence of the adverb in the second sentence makes the verbal sense and the implied agent much more clear, i.e. “someone breaks the windows regularly”. The first sentence is simply describing a state and doesn’t specifically reference a recent action.

Your sentence “the pot was watched” could be interpreted either way depending on the context. Are you saying “the pot is a type of pot called a watched pot”. In that case it’s an adjective. But if you’re answering the question from the chef “did you watch the pots?” “This pot was watched” then it’s a passive construction. “I watched this pot, this pot was watched by me”. “By me” being the implied agent. It depends on what you’re trying to get across.

My question: Couldn’t I just as easily (and sensibly) understand the sentence in #3 as a linking verb (was) + an adjective (watched)? After all, that is how was is used in sentence #1, and how watched is used in sentence #2.
– Yes. Though don’t you understand the sentence without thinking about the terms “linking verb” or “passive voice verb phrase”? (It seems to me that those terms are mainly for linguistics, and are confusing even in grade school, which was the last time I remember hearing the term “linking verb” at all.)

”“Being watched” isn’t an action taken by the pot—it’s more of a descriptor of the pot, isn’t it? It’s the watched pot, the pot that is watched.”
– That doesn’t naturally occur to me when I read “The pot was watched.” If I pondered it, my next thought about it would be more like “someone was watching the pot.”

“When we use the passive voice, aren’t we essentially turning an action (verb) into a description (adjective)? Making the action a passive state of being?”
– If it’s just “to be”, then yes we’re just assigning an adjective.
– Otherwise, not primarily, no. With passive verbs, we’re saying that something happened to something.

“What’s the value in considering passive voice construction as a verb phrase instead of as a linking verb + past participle-as-adjective?”
– Because that’s usually what passive voice is. It is not just giving you an adjective.

Bob was hit by an axe.
This is not primarily saying that bob was given the quality of being-hit-with-an-axe.

“Second, where is the line between when (to be + past participle) is considered the passive voice and when it’s considered a linking verb + adjective?”
– You seem like you understand what the sentence means, so unless you are studying grammar, I really wonder why you are hung up on trying to assign grammatical definitions and have only one be right. They’re both grammatical perspectives, and probably neither of them are of much use to someone who understands English as well as you do.
– You can think of “was tarnished” as either, and be right each way. It doesn’t lead you to think something else is meant by the sentence. The thing about “tarnished” is that the past participle and the condition are commonly used. Many past participles wouldn’t be so natural, particularly with verbs that describe short events and aren’t generally used as past participle states much, such as lambaste, or annihilate, or ridicule, or teach.
– It’s more a question of what the writer means to say and how they are saying it.

Thanks all.

Would it be fair for me to summarize as—passive voice and linking verbs + past participles share the same syntax/pattern, but there’s a semantic distinction between the two? (The distinction being—if the idea is understood as an action done to the subject, then it’s passive voice; if the idea is understood as a description of the subject, then it’s linking + past participle… but the border here can by fuzzy).

@Zaku Yep, I am studying grammar, in order to teach grammar to others, and trying to make sure I understand the distinctions between concepts. I’m not so much hung up on one being right as much as I didn’t understand what defined the border between the two ideas (or whether there was a difference at all). I realize this is probably a pretty pedantic question, so I definitely appreciate the responses!