Three questions about short adjectives

Question 1)

Челове́ческому гла́зу э́то недосту́пно.

In this sentence is недосту́пно an adverb or a neuter short form of the adjective недосту́пный ?

Question 2)

My dictionary often gives only say the masculine and feminine forms e.g.

хороший (-, а)

but sometimes if gives masculine, feminine, neuter and plural forms e.g.

храбрый (-, -а, -о, -ы)

does this mean that is there is no e.g. plural form in those cases where it is not given?

Question 3)

Some adjectives have a short form and others don't. Is there any way to know an adjective can be used in the short form other than just knowing?

спасибо

1. It is the short form, not the adverb form. You can tell this because the word doesn't have any other word to modify. The word is, itself, the predicate of the sentence ("blank *is* blank" scenario)
For it to have been the adverb form, it would've made sense to translate it with a -ly on the end, roughly speaking.

2. It's hard to tell why your dictionary does this.
If an adjective has one short form - it will have all of them. I seem to have a sneaking memory floating in my head of one or two times I've seen an adjective that had only certain short forms. Either way it's either absolutely incredibly rare or it just doesn't happen.
I think your dictionary might do this, perhaps because of some reasoning involving the Spelling Rules; but that theory is hard to defend.

3. Though a universal rule I think is hard to come by, generally, adjectives that are used exclusively as attributive, particularly long and specific ones, will not have (or really need) short forms.
правительственный - governmental
зубно́й - dental
разгово́рный - conversational, colloquial

(tempting as it may be to extract out English -al as a sign of a non-short-form adjective - that doesn't work all the time)

To some extent, if you can't make short forms out of it, it is because you don't need to.

1. Answered in full above.

2. More examples needed, but from the words you presented there it seems like your dictionary does this for some unknown reason as I can easily give you the plural short form of хороший. Хороши.

3. Russian adjectives form three sub-groups. I'm not sure what the exact translations of those groups would be in English, so I'd just use their Russian names:

Качественные прилагательные - they describe a quality that can be a varying attribute of a noun, changing to a varying degree. E.g. Быстрый - Some people can be very fast, some will be even faster than that, some will be just fast enough to be destinguished as fast but no more.

Относительные прилагательные - they describe a quality that cannot vary in any way. E.g. Правительственный or зубной for that matter. Someone or something can be one of those, but they can't be more/less dental, more governmental, etc.

Притяжательные прилагательные - these are just possesive adjectives, e.g. мамин, папин etc.

Of all those groups only качественные прилагательные have a short form.

Thanks iCake

Another couple of examples of 2. -

худ|ой (~, ~а, ~o) no ~ы but i assume this exists - худы e.g. студенты худы?
цен|ный (~ен, ~на) no ~но or ~ны but i assume e.g. эти ценны is possible?

But I get the general idea. Most likely the dictionary only gives forms which it considers irregular or which cannot be inferred.

Thanks for expanding xXHoax's point about which ones can have a short form. I guess the main point is that in most cases it will just work out alright from the sense - i.e. unconsciously I would not try to use adjectives other than qualitative ones in a short form. Also - I think (according to my grammar book) short adjectives are "bookish" - that is not used so much in speech?

спасибо болшой