English grammar has an interesting peculiarity of its verb morphology in that the subjunctive mood governing wishes and counterfactuals of the type headed by if triggers the use of the past tense rather than the present in the copula. Thus the normative pattern is I wish I were in Dixie, but the song goes “I Wish I Was In Dixie,” exemplifying a long-established deviation from the traditional norm whereby the number of the verb to be in the subjunctive can be singular rather than plural, while the tense remains past (rather than present) in both normative and colloquial usage.
The very fact that the number requirement in the copula has been relaxed––but not the tense––is evidence of the weakening of the grammatical force (salience, restrictedness) of the subjunctive mood as a verbal category in the history of English. It is also a sign (in the strict semeiotic sense) that tense is superordinate to number in the hierarchy of grammatical categories participating in English verbal syntax.
Both past tense and plural number are the marked members of their respective oppositions, non-past (present, future) and singular being the unmarked members. The subjunctive mood is also marked vis-à-vis the indicative, and it is this fact that elicits the appearance of the two marked members of the tense and number categories of the syncretic copula, a process called markedness assimilation, whereby marked grammatical contexts are coordinate with (govern the use of) marked units.
The upshot of this analysis for those contemporary speakers who habitually adhere to and insist––as parents offering a model of language use to their children––on the use in the subjunctive mood of both marked members––the past tense as well as the plural number––in the copula is not just a dogged grasping at linguistic straws but a sign that their internalized grammar valorizes not just the semantic but the EXISTENTIAL AND PRAGMATISTIC distinction between wishes and counterfactuals, on the one hand, and all other sentence types utilizing the copula, on the other.