No, it's not a plural; it's the simple past subjunctive of "to be", which is "were" for all persons. See my article from earlier today: Subject inversion in the correlative comparative construction, and my article from November 23rd regarding the subjunctive: "If you explained what you ___ trying to achieve, I would ...".
Here are some examples of the subjunctive although some of them are a bit old-fashioned or archaic or highly formal:
If I were the President of the United States, I would cut
taxes. (both "were" and "would" are simple past subjunctives of "to
be" and "will" respectively.)
If I wanted you to get involved, I would ask for your help.
(both "wanted" and "would" are simple past subjunctives of "to want"
and "will" respectively.)
If I be elected President, I shall cut taxes. ("be" is the simple
present subjunctive of "to be".)
If I were elected President, I should cut taxes. ("were" and
"should" are simple past subjunctives of "to be" and "shall".)
I shall do it if he so want me to. ("want" is the simple
present subjunctive of "to want".)
It's important that they be on their best behavior. (simple
I demand that she answer me! ("answer" is the simple present
subjunctive of "to answer".)
O that she were mine! (simple past subjunctive)
If I had gone to the party, I would have had a great time.
(past perfect subjunctive of "to go")
It's imperative that he have finished college by the time he is
23-years-old. (present perfect subjunctive of "to finish".)
He talks as though he knew everything there is about being in
love. (simple past subjunctive of "to know".)
I wish the subjunctive were used more. (simple past subjunctive)
I pray that the subjunctive be revived. (simple present subjunctive)
The subjunctive is somewhat fancy and formal, but it is still considered good English today; in fact, in some instances, it is considered really good English. It was more popular during the nineteenth and early twentieth centuries than it is today, although it's been experiencing somewhat of a revival since it is quite frequently used in American English, particularly the mandative subjunctive:
I recommend that she not do that.
It is also very popular in literature, especially literature from the nineteenth century and earlier centuries:
"Men's courses will foreshadow certain ends, to which, if persevered
in, they must lead," said Scrooge. "But if the courses be departed
from, the ends will change. Say it is thus with what you show me."
Ebenezer Scrooge, A Christmas Carol by Charles Dickens, 19 December 1843.
"The basis of our governments [sic] being the opinion of the people,
the very first object should be to keep that right; and were it
left to me to decide whether we should have a government without
newspapers, or newspapers without a government, I should not hesitate
a moment to prefer the latter. But I should mean that every man should
receive those papers and be capable of reading them." Letter from
Thomas Jefferson to Edward Carrington, 16 January 1787, Paris,
I hope this might have helped you out. If you should have any questions, please feel free to ask me. Take care and good luck!
P.S. As many people on this forum know, I do not buy into the fabricated irrealis mood that some modern grammarians now adhere to. I go by what it was in Old English and it clearly was the subjunctive in Old English; and just because the language has syncretized over the past 1,000 years doesn't mean it's not the subjunctive anymore:
past subjunctive of "to be" in Old English***: ic ƿǣre (I were); past indicative: ic ƿæs (I was). present indicative:
ic eom (I am); present subjunctive: ic bēo (I be). This runic letter ƿ
is wynn; it makes a w sound. Present indicative of "will": ic ƿille (I
will); past subjunctive: ic ƿolde (I would); present indicative of
"shall": ic sceal (I shall); past subjunctive: ic sceolde (I should).
And these are just a few verbs and only the conjugations in first-person singular; there are thousands of other verbs that also show this, but haven't been posited above. Yes, English has syncretized over the past millennium, but it is still the subjunctive; the only problem is that the language has syncretized so much that one can only see the past subjunctive in the verb "to be" because it is the only verb in English whose past-tense inflections change depending on the person and whether the person be singular or plural.
Now Sungil has asked about the difference in meaning regarding the tenses of the "If I were elected" and the "If I be elected" examples stated in my answer above. First, Sungil, the subjunctive is not a tense; it's a mood; therefore, these verbs are not inflected into a tense, but inflected into a "mood"—the subjunctive mood to be exact. The "If I be elected" example is the present subjunctive and it's very formal bordering on archaic, just to let you know. It differs from the "If I were elected" example insofar as the "I be elected" one expresses a real possibility in the present whereas the "I were elected" one expresses something that is contrary to fact in the present. Apparently, it is possible that I may be elected President, so "if I be elected" President, I shall cut taxes. Since the chances are slim that I shall ever be elected President, I would most likely say, "'If I were elected' President, I should cut taxes," since it is nearly impossible for me ever to be elected President. If I were a politician like Hillary Clinton or Ted Cruz or Mike Pence, it would probably be possible then and the very formal or archaic "If I be elected" example would prevail. In Modern English, the present subjunctive seldom occurs after a subordinating conjunction anymore except in very formal speech and prose normally. It can occur in much more informal settings, but not too often.
In any event, that's the subtle difference between them, Sungil. The present subjunctive in "if" clauses talks about real possibilities in the present and the past subjunctive talks about counterfactual situations in the present.
*** There were three verbs in Old English that formed the Modern English verb "to be".