All I know is the direct answer and it would be it is different because one is your thought and one if like a document
The comma would end up between but and orange.
"You may have an apple or an orange, but you can't have both."
This is because you're coming to a conjunction, which usually has a comma preceding, or coming before, it.
A singular verb would be used with a singular noun (she, Bob, Jane, truck) while a plural verb would be used with a plural noun (they, cars, books).
For example "The truck drives away." Truck being a singular noun and drives being a singular verb.
Another example
"The cars drive away." cars being a plural noun and drive being the plural verb.
