What about the sentenses using "and" or "or", such as:
If you or your staff (have or has) any questions, then please contact me.
then which do you use?
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The answer is "has"- because the subject is singular. It refers only to either "you" or "your staff".
Now, if your subject is " you and your staff" instead of "you or your staff", the correct verb to use is "have".
ex:
If you or your staff ( has) any questions, then please contact me.
If you and your staff (have ) any questions, then please contact me.