What's the reason for the enclosed English grammar usage?

“Were” is the subjunctive mood of “to be.”

Right. In this use it isn’t plural at all. It just so happens that many slots in the table of conjugations are filled by the same form, but their meaning is as different as the difference in meaning between “am” and “is” or “was” and “will be.”

It’s similar to the sentence “I suggest that he go to the hospital.” Normally you’d say “he goes to the hospital” but because this sentence uses the subjunctive, “go” is in the form that looks like the plural, although it’s actually subjunctive singular.

(The distinctive subjunctive in English is also only manifest in a few cases and in many ways, it is dying out).

If I were king, I could change the rules.

As said above, “were” is subjunctive form not the plural indicative form.
“If wishes were horses, beggars would ride” could also be ” If a wish were a horse, a beggar could ride” Note the all-important “if”

Were it not for the excellent explanations already given above, i might have tried my hand at answering this.
Hey, there are some very distinguished linguists here on Fluther!

Should it be possible to think of more examples, someone here would probably do it.

How about ‘I wouldn’t do that if I were you’? Isn’t it something about a hypothetical, rather than actual, situation? Ah, here’s a site that explains it.

“If I were a wealthy man…”

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http://m.youtube.com/#/watch?desktop_uri=%2Fwatch%3Fv%3DPl7BVr36bbs&v=Pl7BVr36bbs&gl=US

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“All day long I’d dibby , dibby dum…....”