Bob didn't come to office today.
I was asked "Didn't you see Bob today?"
Should I say "Yes, I didn't see him today" or "No, I didn't see him today"?
"Didn't you see Bob today?" means that the person asking the question assumes that you saw Bob. The negation works like a tag question, so the sentence basically means the same as "You saw Bob today, didn't you?"
So, the appropriate response should be as if the tag question were omitted.
Speaker 1: any of… "Didn't you see Bob today?" or "You saw Bob today, didn't you?" or "You saw Bob today?" (The three formulations of the question are mostly equivalent.)
Your appropriate response: "No, I didn't see him today."
This ambiguity will trip up even native speakers of English.
The question is leading to a negative answer, it expects the following sentence to start with “No.” In my experience, the answer will start with “No,” regardless of whether the answer confirms or denies the statement in question. Either your “No,” agrees with the negative expectation of the question, or the “No,” disagreeing with the expectatoin of the question.
However, in my experience, if someone just answers “No,” even a native speaker will not be certain enough of that intent and will ask for clarification.
For example:
Didn’t you see Bob today?
No.
‘No,’ you didn’t see him, or ‘no,’ you did see him?
No, I didn’t see him.
or
No, I did see him.
For this reason, native speakers will typically respond with the full intent, “No, I did not see him,” or “No, I did see him,” from the beginning, and avoid the back-and-forth.
Saying “Yes, I did not see him,” might surprise people, as it’s not the expected form, but it won’t confuse. Saying “Yes, I did see him,” will cause less confusion/surprise. But merely answering “Yes,” here is even more ambiguous than answering just “No.”
This reminds me of an old pun played on me by my seniors. They asked me to fill in the blank with either of two options - YES OR NO
..... , I'm not a male!
I was trapped. And, I still don't find the proper option.
But then, think for a while. When such questions are asked, we generally make it clear by adding a clause.
Did you break the glass? ~ No, I did not
Didn't you break the glass? ~ No, I did not
Both the sentences though begin in different way has the same answer. No matter what but from the conversation, it's quite clear that the person in doubt has not broken any glass.
In a similar way, it does not matter how the question is asked, you'll have to answer the way you have to!
Didn't you see Bob today ~ Yes, I did OR No, I didn't.
Did you see Bob today ~ Yes, I did OR No, I didn't.
Note that to avoid ambiguity, you have to add a clause justifying your Yes or No. Good to note as Lucian said that usually Yes is followed by an affirmation. Yes, I did not... is not logical.
You may simply say:
No, or
No I didn’t, or
No I didn’t see him today.