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Why have the subjunctive and indicative converged in Modern English?
Why is it that in the English language, the past subjunctive is the same as the past indicative for all verbs except to be? In other languages that I have learned there is no relation between these two tenses, even in languages similar to English.
There is an interesting article in Wikipedia (http://en.wikipedia.org/wiki/English_subjunctive), where you can see that it hasn't always been so. For example:
In Early Modern English, the past subjunctive was distinguishable from the past indicative not only in the verb to be (as in Modern English), but also in the informal second-person singular of all verbs. For example: indicative thou sattest, but subjunctive thou sat.
As to your actual question, languages evolve in ways not easily understood, and certainly not following other languages. When a certain form doesn't serve the purposes of communication, then it slowly dies away since speakers avoid it and replace it with more functional forms.
Old English had a present and an imperfect subjunctive, inflected for person and number. Those forms have been lost over the centuries, leaving us only with one form, which is identical to the plain form of the verb and which can be identified only in the third person singular, as in It is essential that he leave. It makes no sense to speak of a past subjunctive because, as you say, any such form is indistinguishable from the past tense, and linguists no longer do so. Some might argue that were as in If I were you is a past subjunctive, but that supposes there is a present form, and in modern English there isn’t. The linguists Rodney Huddleston and Geoffrey Pullum go so far as to deny that were is subjunctive at all, preferring the term ‘irrealis were’.
