Why is "Those boys are good" not a correct translation? How are you supposed to know the difference?
I may be wrong, but I believe "Those boys are good" would be "Boni taobi syyri issi."
Since Valyrian is typically SOV (Subject-Object-Verb), this particular sentence can be broken down as "Boni-syyri taobi-issi," or "Those-good boys-are." Translating this into the English SVO word order we get "Those-are-good boys"
Breaking up "Boni taobi syyri issi," however, would give us "Those boys-good-are," which is "Those boys-good-are." Remember that adjectives typically come before the nouns they modify.
Hope this could help :)