Should I use
were in sentences like this:
Who was/were the bride and groom?
In my opinion, both are possible, but I don't know why. What to use if there is
or instead of
The correct answer is:
Who were the bride and the groom?
The past form of the verb TO BE is: I was; You were; He/She/It was; We were; They were; You were. So, Was represents a singular form, while Were is the second person, singular, or a plural form.
In your sentence, the bride and the groom means they, which is plural, therefore you need a plural form for the verb, that is why you need to use were.
If you use OR instead of AND, it means you refer to only one of two people, so you need to use a singular form of the verb: WAS.
However "Who was the bride or the groom?" doesn't sound natural to me. I'd say: "Who was it, the bride or the groom?"
Was is used in the first person singular (I) and the third person singular (he, she, it).
Were is used in the second person singular and plural (you, your, yours) and first and third person plural (we, they). More Info
So based on the above facts, it's
Who were the bride and groom? (It's a Plural form)
What to use if there is or instead of and?
Now, if we say "or" instead of "and", we refer to a merely singular person.
Hence, it will be..
Who was the bride or groom?
Here is why you may think was could be used in this case. I'm going to add a few words in brackets to highlight an implied parallel structure if you use was:
Who was the bride and [who was] [the] groom?
Parallelism is common in lists of two or more items, as it is a way to reduce repetition by suggesting that every item should be read from the first part. It's evident from that reading-by-parallelism that the speaker is asking who the bride is and who the groom is. Compare this to a more straightforward example to see why your singular verb might not be preferred here:
The coach told the players that they should get a lot of sleep and not eat too much.
You don't need to read "that they should" before every statement individually, because it's implied structurally. Your example with was adds a wrinkle because the subject "who" doesn't strongly signal number (singular or plural) on its own. With a plural verb, "the bride and groom" is easily read as a unit. With a singular verb, "who" could be read as a singular subject and "the bride and groom" could be read as two separate people acting, each preceded by "who was."
Now, this may be a more awkward reading because it requires readers to fill in more of the blanks, for reading it as a parallel structure makes the reader work more than just changing it to a plural verb. Furthermore, usage suggests that the phrasing in singular form does happen, albeit more rarely than the plural verb counterpart. So it's not a matter of one version being correct (unless the speaker doesn't have a grasp of singular and plural verbs and intended "who" to be plural but used the singular "was", which goes down a rabbit hole of intent that's difficult to prove), but rather the plural version being more common, more conventional, or easier on readers.
Still, if you said "who was," you'd have an argument for it. One final example of using a singular subject and verb with a noun list in parallel structure:
Her purpose was to impress the ignorant, to perplex the dubious, and to confound the scrupulous.