"His mother is short."
Translation:Pōkole kona makuahine.
"Kona makuahine pōkole" is not a complete sentence. It just means "His short mother." When "pōkole" follows a noun, it is an adjective describing that noun. When used at the beginning of a sentence, it can be a "stative verb" which works like a verb in Hawaiian although it looks like an adjective in English. They wanted a complete sentence here, and remember that there is no verb "to be" in Hawaiian, so this is one pattern that replaces the need for "to be."
ʻO ia pōkole kona makuahine. Also is marked wrong. I thought it might translate as "She short his mother." Which sounds unfinished in English but reminded me a bit of the Yoda-wording mentioned in other posts. What did I say, is there some grammatical rule I ignored? Mahalo.