German help (Du vs Ihr)

So I got this one wrong today, I think the sentence i used is gramatically correct. (Please correct me if wrong). How do I learn when to use "du" for you or "Ihr" instead (and their associated verbs)

Thanks in advance guys :)

So I got this one wrong today, I think the sentence i used is gramatically correct. (Please correct me if wrong). How do I learn when to use "du" for you or "Ihr" instead (and their associated verbs)

Thanks in advance guys :)