sudanglo:1. In principle (for consistency) any preposition could stand before an infinitive in Esperanto, so 'per rigardi' is a theoretical solution. But in practice the set of prepositions that Esperanto allows in this position is limited and as yet 'per' hasn't joined this exclusive club.

So either you have to say mi komprenas ĝin per rigardo (rigardado/ekrigardo/trarigardo), or as Mutusen suggests by use of the adverbial participle.

2. Although the norm is to use the adverb, one can occasionally come across sentences where the adjective is clearer than the adverb when qualifying Verb Phrases without a referential noun/pronoun for the adjective to connect to. There are, I believe, Zamenhofian examples of this deviation from the norm but I can't recall any specific example, or make up a good one myself.

Certainly in elliptical sentences it seem possible. Ĉu ebla, iri per tramo? (Ĉu eble iri per tramo? feels more like a tentative suggestion). However the full sentence has to be Ĉu estas eble iri per tramo?

Zamenhof used haphazardly all possible combinations, i.e. (a) predicate noun with -e over verbal subject, (b) predicate noun with -a over verbal subject, (c) predicate noun with -e over verbal object, and (d) predicate noun with -a over verbal object. Here some examples:

A: Resti kun leono estas danĝere.
B: Estus tre utila tute precizigi kaj klarigi la rolon de la Oficejo.
C: Nek Snitchey nek Craggs trovis utile batali malkaŝe kontraŭ la fluo de tiu ĉi kolero.
D: Ni trovis necesa doni la sekvantajn klarigojn.

The now widely accepted norm to use predicate adverbs instead of predicate adjectives whenever the predicate refers to a verbal phrase, seems to be postzamenhovian. Maybe the examples B and D should be deemed archaic? I don't know.

On a side note: The agreement rules in Esperanto are in my opinion the most complex and most difficult part of the grammar.

Where did you find B and D, Nornen? I was wondering if they were from earlier or later Zamenhofian texts.

If I had to make a recommendation, it would be to follow the norm, unless using an adjective instead of an adverb aids in understanding - avoids ambiguity. I don't see that any proscription on using the adjective should be absolute.

I find D. pleasing, elegant.

What stumped me was trying to recall a sentence I had seen somewhere that almost changed meaning if the adverb were used (and making up an example of such a sentence).

B: PAG 1985:232, §169, Rim. I, marked as Zamenhovian
D: PAG 1985:236, §173, Rim. I b, marked as Zamenhovian

nornen:

Tempodivalse:

Fenris_kcf:You did not get my point. IMO "bon" refers to the left out "afero" and must therefore be an adjective, not an adverb.

The adverb ending in this case is more appropriate. "Estas" is in this case an impersonal verb and thus must be modified by an adverb, similar to the constructions Estas varme or Estas pluve (i.e., no overt subject).

Correct conclusion, wrong argumentation.
In "Estas bone legi librojn", the predicate noun "bone" has the adverbal ending because its antecedent is verbal. Adjectives referring to verbs or VPs always take the adverbal ending in Esperanto, no matter whether they are attributive (legi bone) or predicative (legi estas bone).
This has nothing to do with impersonal verbs or lack of overt subjects, nor does "bone" modify "estas".
In "Estas bone legi librojn" the verb "esti" is not impersonal and there is indeed an overt subject. The subject of "estas bone" in this case is "legi librojn".

As a reference see the Fundamento: Resti kun leono estas danĝere.

My apologies, I was thinking of something else entirely, but "impersonal verb" somehow was the term that popped up in my head - and I got derailed from there. I suppose it's incorrect to say that "bone" modifies "estas", but that's the way I think of it, to resolve uncertainties about the correct part of speech to use - put in other words, "if it goes with a verb, it's an adverb, if it goes with a noun, it's an adjective".

D: Ni trovis necesa doni la sekvantajn klarigojn.

In Russian, at least, you would use an adjective here for the predicate, not an adverb (My sochli neobhodimym dat' ...) Using an adverb would result in something like, "We necessarily found...", (which, in this instance, would sound strange). I would parse the alternative Esperanto sentence Ni trovis necese... in the same way. I don't think the adjective and adverb are interchangeable here.

Interesting point about what happens in Russian.

Whilst preferring ni trovis necesa doni ..., I think it is difficult to make it stick that in Esperanto the adverb would change the meaning to 'we necessarily found'.

However, because in Esperanto adverbs can come before and after the verb they qualify - ni rapide trovis, ni trovis rapide - the possibility that the 'necese' might qualify 'trovis', rather than what follows, sort of hovers at the fringe of consciousness.

A better example of potential for ambiguity would be:

Estas nekredeble stulte fidi je la estonteco de la Eŭro

[estas nekredebla, stulte fidi .. or estas nekredeble stulta, fidi]

Edit: But in changing the order to make the meaning explicit , I am tempted to stay with the traditional adverb usage (in both 1. and 2.).

1. Senpense fidi je la Euro estas ne plu akcepteble
2. Ankoraŭ fidi je la Eŭro estas ne-kredeble stulte

Tempodivalse:

D: Ni trovis necesa doni la sekvantajn klarigojn.

In Russian, at least, you would use an adjective here for the predicate, not an adverb (My sochli neobhodimym dat' ...) Using an adverb would result in something like, "We necessarily found..." I would parse the alternative Esperanto sentence Ni trovis necese... in the same way. I don't think the adjective and adverb are interchangeable here.

But the adjective in Russian appears in the instrumental case (as a predicative should do). And slavic instrumentals often appear als adverbs in Esperanto (mi batas martele).
For reasons of simplicity ("everything that is neither noun nor pronoun is accompanied by an adverb" ) the use of an adverb is quite logical here, so it is no wonder that after a short period of hesitation the use of adjectives in such cases disappeared.

Kirilo81:

Tempodivalse:

D: Ni trovis necesa doni la sekvantajn klarigojn.

In Russian, at least, you would use an adjective here for the predicate, not an adverb (My sochli neobhodimym dat' ...) Using an adverb would result in something like, "We necessarily found..." I would parse the alternative Esperanto sentence Ni trovis necese... in the same way. I don't think the adjective and adverb are interchangeable here.

But the adjective in Russian appears in the instrumental case (as a predicative should do). And slavic instrumentals often appear als adverbs in Esperanto (mi batas martele).
For reasons of simplicity ("everything that is neither noun nor pronoun is accompanied by an adverb" ) the use of an adverb is quite logical here, so it is no wonder that after a short period of hesitation the use of adjectives in such cases disappeared.

Fair enough. I was simply thinking of what might have prompted Z to use the adjective here, and the Slavic predicatives immediately came to mind. I would agree that if you generalise grammatical rules, then the adverb makes sense - but due to my Russian influence, something feels "wrong" about the -e form.

I think the problem is that in EO there is no immediate way to tell which verb an adverb modifies, unlike adjectives and nouns (which have case, number agreement), so we are left with only proximity to the verb. When an adverb sits between two verbs, we are left to rely on context. In the "Ni trovis necese..." example, the context is fairly obvious, so this was not the best example of potential ambiguity (e.g., if someone in Russian had said neobhodimo instead of neobhodimym, the intended meaning would still be readily understood). But in other contexts it is not so. And this bothers me - the inability to safely generalise on observed usage, which is what makes EO so easy elsewhere.

so it is no wonder that after a short period of hesitation the use of adjectives in such cases disappeared

Yet, Kirilo, there are cases (eg double adverbs) where using the adjective helps understanding.

And you could perfectly well argue that ni trovis necesa .. is just an elliptical form of ni trovis tion necesa .. and quite logical. [Esperanto not infrequently drops a tio(n)]

something feels "wrong" about the -e form

Trust your feelings. It isn't that the adverb is wrong so much as that the adjective in some cases is better/more elegant - more clearly conveys the meaning.

In Esperanto clarity is a more important principle than blind devotion to systematicity.