In this case, azantys is in the nominative case and taobe is in the accusative case. That means azantys is the subject while taobe is the direct object, which is shown by the change in suffix from taoba to taobe (note the change from -a to -e). Because of this, the following both mean "The knight sees a boy.":
Azantys taobe urnes.
Taobe azantys urnes.
Notice that since taobe has the suffix change, it's in the accusative case no matter the order of the nouns. In addition, High Valyrian has no definite or indefinite articles, meaning that you have to use context clues. However, for the sake of translating, Duolingo accepts both "The knight sees the boy." and "A knight sees a boy." as valid translations for this sentence.