"Is the mountain good?"

Translation: Blēnon sȳrior issa?

[adi_enj]

how to know which 'good' to use

[xPsGoDWh]

It depends on which type of noun it modifies. In this case, "blēnon" is a terrestrial noun in the singular. If it was "blēna", it would be "sȳriar". It's the same for aquatic nouns too, but for solar and lunar nouns, it's sȳz/sȳri, see https://wiki.dothraki.org/High_Valyrian_Adjectives#S.C8.B3z-type