I was a bit surprised at first when I discovered that Chinese doesn't have single words or expressions for "yes" or "no", and that instead we must use the verb from the question to either confirm or deny. The difficulty is not insurmountable if we can understand the verb from the question, but there seems to be exceptions, and this brings me to the object of the question.
I have this sentence and I need to modify as a yes/no question and then answer both positively and negatively:
Wǒ mèimei lái Mĕiguó.
Nĭ mèimei lái Mĕiguó ma?
I answered with "来。／不来。", but looking at the key of the exercise, enough surprisingly I noticed I was wrong, since I should have answered "是。／不是。".
Why is 是 used here instead of 来？ Is this event contemplated in the grammar, I mean, are there rules/guidelines to know when we can use 是 even if it's not the verb used in the question?