Yes -- there is a full stop "." at the end, making this an entire sentence.
So here it expects you to translate it as a sentence: "he runs" or "she ran" or "they will run", for example.
If the exercise had been just qet (a single word, no punctuation), then "run" would have been a good translation; but this one is qet. (including punctuation).
Well the issue with that is that "Run." is a sentence. It's usually said with an exclamation point, but it is nonetheless a sentence. If you want the answer to be "He ran", then instead of being greyed out in the drop-down box, the He/She/It/They should appear in standard text, as though they are part of the definition.
True, "Run!" is a sentence -- however, it has a different meaning from "He runs" or "He ran".
"Run!" is a command, and commands in Klingon need a special verbal prefix -- thus, *qet. cannot be the command "Run!".
I'll have a look at the hints and see whether I can improve them.
No -- it's a complete utterance, but not a complete sentence.
It's in the same group of utterances as "At the zoo." or "Yes.", which are complete in and of themselves in the context of an answer to a question, but are not complete sentences. (They have no verb, for example.)