"When are you going walking?" was marked as incorrect. Should it be accepted?
I feel like it should as well, although spazieren seems to have the slightly more specific meaning of to walk for a walks sake, rather than to generically walk. as a native English speaker, I would probably not make the distinction except to accommodate certain stylistic speech. otherwise I easily could equate "when are you going walking" and "when are you going for a walk." Slightly different ideas in German I guess.
Why do you think "We are going walking" would be correct?
You did not translate the Wann nor the Sie in the German sentence, and I don't know where the "we" in your English sentence comes from.
It's not really wrong but no one would say that. Just sounds weird. "When are you going for a walk?" sounds appropriate.
The -en ending not only indicates plural. In this case:
*the -en ending of "gehen" comes from the conjugated form "Sie gehen" (as the singular formal form)
*the -en ending of "spazieren" comes from the infinitive form.
Maybe you are confused because of the word order. Notice that in a question the subject and the verb are reversed, so that in this case the conjugated verb is "gehen", not "spazieren".
If this wasn't a question it would be: "Sie gehen spazieren."
Yes, you are wrong. Capitalised Sie does not mean "they"; it means "you".
I am not a native English speaker. But i think "When you are going for a walk?" Should be accepted.
No; that is not correct English word order for a direct question.
“I know when you are going for a walk” (indirect question) but “When are you going for a walk?” (direct question).