Why does "past tense" and "subjunctive" have the same form?
Are there any references or research papers?
...the name past subjunctive refers to the form of the verb rather than its meaning; it need not (and in fact usually does not) refer to past time.
English subjunctive - WikipediaThe form is called the present subjunctive because it resembles the present indicative in form, not because it need refer to the present time. In fact this form can equally well be used in sentences referring to past, future or hypothetical time (the time frame is normally expressed in the verb of the main clause.
so why we need two forms of subjunctive？（"Present subjunctive" and "past subjunctive" ）"Present subjunctive" and "past subjunctive" refer to the form of the verb rather than its meaning.
See also here, where a very similar question of yours has been answered. The answer also explains why past subjunctive is used in the above sentence:"If I were you. I would..." means "if I were you now. I would..." It’s present tense.
Why does “past subjunctive” need to be used here？Why not use present subjunctive？
Thank you !
Past subjective ... expresses hypotheticalness
While I mainly agree with your analysis here, I think what really holds these forms together is that they express deontic modality, which is a broader concept than just attempting to cause change or make imagined events real. For example, in the sentence 'The bad weather necessitated that he cancel the expedition', there is no attempt to change something. The thing that caused the change is inanimate.The past & present subjunctive have bad names.
"Past" subjunctive is used merely to describe what an imagined alternative is or was, without any attempt to cause or change it. "Present" subjunctive is used to describe an attempt to change an outcome or cause something to happen, to make an imagined event real. Notice what the example sentences here depict somebody doing, which the subjunctive verbs describe:
insist (trying to get somebody to do something the insister wants)
ask (again, trying to get somebody to do something the asker wants)
recommend (again, trying to get somebody to do something the recommender recommends)
brake (this time trying to get a car to do what the driver wants)
be a necessary condition for bail (withholding bail to get somebody to do what is required)
This is not passive – just the verb 'be' with an adjective. I'd say it's ambiguous as to whether the speaker him/herself desires the action to be carried out.The only present subjunctive example with a bit of ambiguity in its meaning is "be desirable", because it uses passive voice to hide who desires it, but, the way it's actually used is to express a desire of the person who is speaking and try to get the listener to do what the desirer desires.
Indeed, synchronically the labels are to be understood purely morphologically and not semantically. In some languages the tense labels have therefore been dropped like in German where they are simply called Konjunktiv I and Konjunktiv II. The logical evolution leading to the current use is still fairly transparent. The Germanic subjunctive in general evolved from and expresses the optative mood. How optative can transform to express hypotheticalness, present as expressing a possible scenario and past as an unreal or at least unlikely one, I find relatively easy to see.The past & present subjunctive have bad names.
"Past" subjunctive is used merely to describe what an imagined alternative is or was, without any attempt to cause or change it.