When shopping at a supermarket, I think that the clerk often asks,
レジ袋は大丈夫ですか？
"Do you need a plastic shopping bag?"
Have you ever seen a Japanese person answering "yes(はい)" but not getting a bag?
○"Yes" means that there is "no problem", "OK".
Let's check with this clerk by adding the omitted Japanese to the Japanese conversation.
First of all, the cashier's words are exactly as follows.
Cashier：
(あなたは)レジ袋を(購入しなくても)大丈夫ですか？
Are you okay without buying a plastic shopping bag?
So, literally, the answer "Yes I am" means "no need to buy" or "unnecessary".
However, the interpretation may be different.
In the previous explanation, I put "without buying " as the part omitted in the remark. However, in some cases, the recipient of the conversation complements different words, resulting in a completely different interaction. Let's look at an example.
Cashier：
レジ袋を(お付けしても)大丈夫ですか？
Is it okay to attach a plastic shopping bag?
In this case, the customer answers "YES" when it is okay to attach a plastic shopping bag, and the customer answers so when he / she wants it.
It is a difficult part for Japanese people.
「レジ袋は大丈夫ですか」 can be interpreted in two ways, depending on how the abbreviated word is supplemented. Translated literally, it's "Do you need a plastic shopping bag?", but that's not all.
Even Japanese people don't understand the meaning of YES(はい), so I sometimes ask the other person twice.
If you don't understand in the conversation on the spot, it won't be rude to ask back, so let's check it properly!
On the contrary, when you answer the questions of the cashier in Japanese, You can add an explanation after "YES(はい)" or "NO(いいえ)" to make it easier to understand.
Example
○"Yes, I need it."
はい、必要(ひつよう)です。
はい、お願いします。
○"No, I don't need it."
いいえ、要りません
はい、大丈夫です。