A native speaker would understand you, but would think it an odd question structure. "See" is an involuntary action, as compared to "watch" which is a purposeful action. "Do you see the cow?" "Are you watching the cow?"
This order reversal of subject and verb makes the sentence a question. E.g. You have a dog. Have you a dog? (Unlike English, there is no "do" verb in German. In English "do'" is normally used to make a question. E.g.: You have a dog. DO you have a dog?)
Probably because "Siehst du die Kuh" is in the present tense, and your answer is in the past tense.
You want me to teach you all the German tenses in one comment? Look, at this stage, Duolingo has only taught you the present tense. The question is in the present tense. Answer in the present tense, which Duolingo has taught you. That's it. You'll learn the other tenses later.
I guess I'm going to take this as why. We're in the "present" lesson afterall i guess.
I thought she said who and i was think what german word sounds like who???? I didnt hear the "k" sound