"The big thrones are good."
Translation:Rōva dēmalia sȳriar issi.
I need some help. Can somebody explain to my why and when you use the different types of "good" for example in this sentence it's "syriar" (I don't know how to do the accent, I'm sorry) and in other sentences its "syrior". Also (bit off topic) why and when do you use the different types of "my", for example, you have: "nuha, nuhys etc". I'd be really happy if someone could explain this to me!
I'll try to answer your question as well as I can; feel free to ask for clarification if it doesn't make sense.
In High Valyrian, adjectives must agree with the nouns they modify. They agree in terms of gender, case, and number. So, in the sentence ñuha kepa ñuhe muñe jorrāelza, the noun kepa "father" is Lunar (gender), singular (number) and nominative (case). For that noun, ñuha has the -a ending. When describing muñe "mother," that noun is in the accusative case (it is the object of the verb), so ñuha becomes ñuhe to agree with it.
HV has four genders, which should not be confused with human gender. Typically, the gender of the noun is apparent from the nominative singular ending: Lunar nouns end in vowels, Solar nouns end in -s, Aquatic nouns end in -r, and Terrestrial nouns end in -n. The ending of the adjectives change to reflect the nouns. So, ñuha would become ñuhys azantys "my knight" or "ñuhor lōgor "my boat" or ñuhon lenton "my house." Though these are simple examples (some adjectives, such as sȳz, are the same for Lunar and Solar nouns and Aquatic and Terrestrial nouns), you can see how this sort of agreement works.
Similarly, adjectives reflect whether the noun is singular or plural, as in rōvon dēmalion "the large throne" versus rōva dēmalia "the large thrones". My earlier sentence displayed the agreement in noun case with ñuha kepa vs ñuhe muñe.
There are three classes of adjectives, so adjectives will change their endings according to one of three patterns (with some exceptions that you've already seen, like sȳz). You can view tables illustrating them at the wiki, which will hopefully make things more clear. Though it may seem overwhelming at first, just keep practicing and it should start to feel natural.
TL;DR: sȳriar is used because dēmalia "thrones" is a plural Terrestrial noun. If it was the singular dēmalion "throne" it would be sȳrior. See https://wiki.dothraki.org/High_Valyrian_Adjectives