This is the story of one of my students. When he visited a Japanese friend, the Japanese friend asked him "Koohii wa ikagadesuka ?" As he wanted to drink it, he said "Hai ii desu ." He had been waiting for the coffee. But no coffee was served. He thought "Hai ii desu" meant "Yes, please." But in this case it means "No thank you." He should say "Onegaishimasu."or "Hai,arigatoo." If the question is "koohii de ii desuka.", "Hai,ii desu." means "Yes, please. "Ii desu." is such a vague expression that many students seem likely to misunderstand it. Please remember "Iidesu."means either "Yes, please." or "No, thank you."depending on the situation. Be careful when you say "Iidesu."