Because ñuhī can be placed before or after the noun, how are you tell whether the sentence is "The masters eat my grapes" or "my masters eat the grapes"?
In this particular case ñuhī can only refer to the accusative.
ñuhi would be the nominative. The charts can be checked here: https://wiki.dothraki.org/High_Valyrian_Adjectives#Class_I
Both nouns are lunar plurals. They can be looked up here: https://wiki.dothraki.org/High_Valyrian_Vocabulary#A
In other sentences ambiguity may exist [in the adjectival word in such a way that it can attach to either the nominative or accusative].
Because both nouns are lunar, the adjectives both take the ī ending. I still don't understand why ñuhī belongs where it does?
"The grapes eat my masters" is a possible translation by my reading.
[accusative/object = Āeksia ñuhī ] [nominative/subject = avera] [verb = ipradis.]
It would use OSV word order, which should be fine - changing the word order is stated as licit in https://www.duolingo.com/skill/hv/Basics-2
Given some of the other amusing sentences in the High Valyrian course, the course contributors might accept "The grapes eat my masters." as an alternative translation.
But unfortunately not "My masters eat the grapes." Which, as a postpositive with a short i would need to be: "Āeksia ñuhi avera ipradis." With the ambiguity that it could also mean "My grapes eat the masters."
The difference is that whatever ñuhī/my with the long ī attaches to, necessarily becomes the accusative/object of the sentence. Which is possible here because "Āeksia" and "avera" can both be either the nominative plural or the accusative plural. https://wiki.dothraki.org/High_Valyrian_Noun_Declensions#Lunar-type:_p.C4.93ko
[I think I have the above right, but it's probably worth checking via the links. Also, I'm unsure how often High Valyrian deviates from SOV, and how often it allows the adjective form of the possessive pronouns to go after the noun they possess.]