Bruce Mitchell tells us in ‘An Invitation to Old English and Anglo-Saxon England’ that Old English had a Present Subjunctive and a Preterite Subjunctive and in each there was a singular form and a plural form. For example, singan (to sing) had Present Subjunctive singular singe (identical to the first person singular of the Present Indicative) and plural singen and Preterite Subjunctive had singular sunge and plural sungen.
As Mitchell writes, ‘the subjunctive is now largely obsolete’, and, as David Crystal writes in ‘Rediscover Grammar’, ‘The subjunctive is used very little in modern English’. Indeed, English has never had a full set of subjunctive forms which serve no other purpose in the way that, say, Latin and French (at least for être and avoir) have. It survives in English in formulaic expressions such as God Save The Queen and Heaven Forbid. These are unlikely ever to change, although the same sentiments can be expressed as May God Save The Queen and May Heaven Forbid. It is also present in what is known as the mandative subjunctive. There, it occurs after verbs of proposing, suggesting and recommending, as in The directors recommend that, after many years of loyal service, he retire and I propose that she offer an apology. Here, too, it is possible, at least in British English, to say instead The directors recommend that, after many years of loyal service, he should retire and I propose that she should offer an apology.
Most grammarians also see the subjunctive in conditional sentences such as I’d tell the truth if I were you and If he were a gentleman, he’d apologise’, where it expresses a hypothetical or unreal meaning, even though it is evident only in the first and third persons singular, where were takes the place of was. However, was is sometimes mandatory, as in If I was rude, I apologize. That has a different meaning from If I were rude, I would apologize, but even there, little seems to be gained by not using the indicative If I was rude, I would apologize. Those who argue otherwise must explain how we are able to differentiate between the two meanings in the second person and in the first and third persons plural. (Those who also wish to preserve the mandative subjunctive must answer a similar question.)
I had a reason for beginning the previous paragraph with the words ‘most grammarians’. The authors of ‘The Cambridge Grammar of the English Language’, Rodney Huddleston and Geoffrey Pullum, describe the use of were in the first and third person singular not as a subjunctive form, but as ‘irrealis’ were. In a sentence such I was rude to you yesterday and I apologise’, was has a temporal meaning, but in If I was rude, I would apologize’, even though it takes the same form, it has a modal one. It is certainly possible to replace was in the second sentence with were, but doing so makes the sentence more formal. It does not make it any more grammatical.