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