> When you say "We can be even more specific than that: the verb is always in -en form," you're also contradicting Pullum when he points out that the present participle is also used to form certain passive clauses. An example of this is also given in the Wikipedia article, "I saw John eating his dinner."
(a) Yes, Geoff Pullum identified a use of the -ing form that is plausibly called passive. I hadn't looked at that before writing my comment.
(b) But you didn't. "I saw John eating his dinner" is not an example of that usage, and is not plausibly called passive in any sense. If you pulled that out of the wikipedia article you just linked, you should have seen the annotation of that very example noting explicitly that the participle is active.
> for example, https://en.wikipedia.org/wiki/Participle, which draws no distinction between "participle" and "participial form"
This isn't even true. Look at the opening sentence:
>> In linguistics, a participle (from Latin participium 'a sharing, partaking'; abbr. PTCP) is a nonfinite verb form that has some of the characteristics and functions of both verbs and adjectives.
Or here:
> The linguistic term, past participle, was coined circa 1798 based on its participial form, whose morphology equates to the regular form of preterite verbs.
We have a distinction between the participle and the form of the participle. The participle is, in this analysis, named after the form, though there is room for debate on that point.
Compare the sentence I've been there before", where been* is in a participial form but is not a participle† and, obviously, displays no characteristics or functions of adjectives.
The second (and final) mention of "participial forms" does confuse the two concepts:
> Some languages have extensive participial systems but English has only two participial forms
This is a conceptual error; for example, Latin has more participial forms (4, or if you really want to stretch it 48) than English, but its participial system is less extensive.
> I think you're mistaken. I'm not sure exactly how, because I'm not sure exactly what you're trying to express, but I'm sure that sufficient study of sources like those will make it clear to you.
You talk a surprisingly big game for someone who doesn't even claim to know what he's talking about.
† There is a school of formalism within linguistics that says that grammatical categories don't exist within a language unless they are reflected in the language's inflectional system. The best-known idea from this school is that English has a past tense but not a future tense. I don't find this plausible; compare https://en.wikipedia.org/wiki/Lexical_aspect , or just the general concept of an isolating or agglutinative language. On this analysis, but only on this analysis, you could say that in the phrase I've been there before, the token been is a participle. You'd have a lot of trouble explaining why it's a participle, though. On any other analysis, it's part of a finite verb, and by definition cannot be a participle.
But by that school of analysis, you would also have to say that English has no passive voice. (Except in the edge case using -ing forms.) And you run into some very awkward problems more or less immediately; when you look at the sentence "I'm going out for the evening", you have to claim that this sentence is fundamentally about being (the finite verb) rather than going out.