Which of these sentences is correct, and why?
"What if the Moon was a Disco ball?" or
"What if the Moon were a Disco ball?"
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We use the preterite (past tense form) when expressing a counterfactuality.
But when it's a form of the verb to be, we can use "were" in place of it.
This form is known as irrealis were. It isn't used for marking tense; it's a mood form indicating that it conveys a degree of remoteness from factuality.
The choice between were and was is a matter of style: were is somewhat more formal than was.
Huddleston and Pullum, A Student's Introduction to English Grammar
Use were (instead of was) in statements that are contrary to fact.
In your sentences it should definitely be:
"If + were" expresses the subjunctive mood, which refers to wishes and desires and is known as a "non-factual" mood.
If you're mentioning a possibility or a probability, a chance that something could be, use "was". Also, if the condition is in line with the facts, use "was".
However, was has become so prevalent that it's worth considering that it may become officially accepted at some point. It's not incorrect to use "was" instead of "were" in casual English, however, strictly grammatically it is incorrect. It's one of those cases like "who to follow" instead of "whom to follow" where the former has become casually used and so common even though it's not grammatically correct.
You use "were". Conditional statements that are contrary to fact take the past subjunctive form in the protasis of the if-then clause. All past subjunctive forms in Modern English are equivalent to their past indicative forms except the verb "to be", which takes "were" in all persons, whether they be singular or plural (present subjunctive "they be"). It is true that in informal English, one will hear "was" said in situations like the one above wherein "were" is grammatically correct although it is often not spoken anymore. This is like the rule "It is I" versus "It's me". It's very formal bordering on humorous and thespian to say "It is I" when everyone says "It's me", but one should never write "It's me" in an English paper. It's a big no-no. So here are some examples of the subjunctive:
"What if the Moon were a disco ball?" ("were" is past subjunctive of "to be") "What if the Moon talked?" ("talked" is past subjunctive of "to talk") "What if I blew the Moon up?" ("blew" is the past subjunctive of "to blow") "If this be treason, make the most of it!" (Patrick Henry's "Treason Speech" to the House of Burgesses in Virginia, May 29, 1765; "be" is the present subjunctive of "to be" in this example.)