Does this conversation sound natural?
A: Can you tell me how to restart this machine?
B: Sure. It's easy. Open the top cover and press a small green button that is located on the back of the top cover.
Should it be "press the small green button" instead of "press a small green button" because there is only one small green button on the back of the top cover?
B did not know the existence of the small green button until A told B. Therefore, I'm wondering if either "press a small green button" or "press the small green button" will do in this case.
English grammar books published in Japan explains:
"This is the doll that he made yesterday," suggests that he made only one doll yesterday.
"This is a doll that he made yesterday," suggests that he made more than one doll yesterday.
Nevertheless, can I use "This is a doll that he made yesterday" in the following cases?
1. I do not know how many dolls he made yesterday (he might have made one doll or more than one doll).
2. It does not look like a doll (because he is clumsy and the person I am talking to said, "What's this waste cloth?" In this case, it does not matter how many dolls he made yesterday.)