Interesting how adjectives change depending on case, gender, and number. For example, because "Riña" is singular, it's "sȳz", but if it was "girls", it would be "sȳri". Have a look here for more information: https://wiki.dothraki.org/High_Valyrian_Adjectives#S.C8.B3z-type
Found this table by looking up "good" here: https://wiki.dothraki.org/High_Valyrian_Vocabulary
how do you pronounce the Y? is it just whyyyyyyyy (sounding like why)
From what I understand, it is pronounced like the German "Ü". Read the tips and notes for that.
Yes, technically it can. This is the hardest part of English-speakers learning Latin and other Indo-European languages is that these languages are SOV, that is, Subject Object Verb. Since this is a simple sentence, you can assume that "The girl" // "good" // 'is' can be turned into "The girl // 'is" // "good" and therefore, because the girl is good, we infer that she is a good girl, if that is the context in which you wish to apply this sentence. But, I would advise against making that assumption until you thoroughly understand what the sentence LITERALLY means. Then, we can apply it to our own language structures and begin to understand how that meaning is being used. I hope that made as much sense as it did to me! Haha
No because it contains the word "girl" in the sentence. However, it could be translated as "The girl good she is" and so be "The girl she is good", but that isn't really correct english so it becomes "The girl is good". Hope that helps.