> Can native speakers of a language even think they have deponent verbs without having another language to compare to?
Yes: it’s all based on the syntax of the single language in question. Latin has very distinct active and passive conjugations [0]; we know they’re active and passive because of the effect they have on the case assignations. Deponent verbs are those which cannot occur in the active conjugations.
More generally, categories like ‘deponent’ have to be justified language-internally. If you can only identify a category with reference to another language, it’s probably not of any relevance in the grammar of the language you’re studying [1].
> Deponent verbs are those which cannot occur in the active conjugations.
That makes more sense. I learned deponent as "Active meaning, passive conjugation". It's only the "meaning" part that I think is subjective and/or relative.
Well, to be honest I’m not quite sure… I never studied Latin. But ‘active meaning’ means very little, as far as I can tell, precisely because it can differ so much even within a single language, to say nothing of how it varies between languages.
>> Deponent verbs are those which cannot occur in the active conjugations.
> That makes more sense.
It's also not quite correct; Latin deponent verbs don't have finite active forms, or active infinitives, but they may have active participles.
As to whether it makes sense to distinguish "active meaning" from the form of the verb...
There is an important piece of Latin syntax exclusively available to deponent verbs, which justifies the description of "active meaning".
Latin has three tenses, past, present, and future. An ordinary Latin verb has a full set of active and passive forms for each of two aspects in each tense.
But a Latin verb only has four participles. They are traditionally known as: present active, perfect passive, future active, and future passive. This is a far cry from the twelve you'd expect. (3 tenses × 2 aspects × 2 voices.) We'll ignore aspect, but the fact that the active voice is missing in the "past tense" in Latin's participial system has a very interesting consequence.
The Latin verb sequor means "follow". It is deponent, and you might choose to think of it as meaning "be led". However, its perfect participle secutus might mean one of two things:
1. It could mean "having followed", a perfect active participial form which is not available to non-deponent Latin verbs. On the analysis where there's no such thing as "active meaning", this is the only interpretation that it should be possible for secutus to have. If sequi [present passive infinitive] means "to follow" and not "to be followed", then secutus [perfect passive participle] should mean "having followed" and not "having been followed".
2. But in fact secutus might also mean "having been followed", passivizing the meaning of the already-grammatically-passive verb. This strongly suggests that native speakers of Latin thought of sequor in terms similar to "passive form, active meaning".
The other reason you might think of deponents as having "active meaning" is that they take direct objects. This is a weaker line of argument, since it's possible for a verb to take multiple direct objects, so there's no real problem with the passive form of such a verb (with two original objects) promoting one of them to subject and keeping the other one as an object.
Compare English "I was given a gift", where I is promoted from its position as primary direct object in "[He] gave me a gift", versus "A gift was given to me", where the only direct object of "[He] gave a gift to me" is promoted.
Yes: it’s all based on the syntax of the single language in question. Latin has very distinct active and passive conjugations [0]; we know they’re active and passive because of the effect they have on the case assignations. Deponent verbs are those which cannot occur in the active conjugations.
More generally, categories like ‘deponent’ have to be justified language-internally. If you can only identify a category with reference to another language, it’s probably not of any relevance in the grammar of the language you’re studying [1].
[0] There’s a nice table in https://en.wikipedia.org/wiki/Latin_conjugation#Regular_conj...
[1] https://doi.org/10.1017/CBO9780511810206.006 is an excellent overview of this, though unfortunately paywalled.