Hellow Zewu
I'm an English learner from Japan. I am afraid I don't know much about English enough to answer such questions as those you are asking. But allow me to put my two cents worth.
First of all, let me define some grammatical terms. In English, people usually say 'subjunctive mood' for 'conjunctive mood' and 'past subjunctive' for 'imperfect conjunctive'.
Now your questions.
Basically English verbs have two tenses and three moods. The tenses are present and past, and the moods are indicative, subjunctive, and imperative. This basic frame of tense-mood relations has not changed through history. As you have already known well, in current English, the verb form for the present subjunctive is the same as the infinitive for all persons. The verb form for the past subjunctive is the same as that for the indicative past except in the case of 'be', whose past subjunctive is 'were' for all persons. One of the features of the subjunctive mood is that the verb form does not vary with persons, while the indicative mood does vary. This was true even in the Old English (the English before the Viking age), though there was a slight difference between singular and plural. The verb tellan (=tell), for example, conjugated as below.
(EX) tellan (to tell)
Ind/Pres (Sg) 1. telle 2. tellest 3. telleth (Pl) 1. tellath 2. tellast 3. tellath
Ind/Past (Sg) 1. tealde 2. tealdest 3. tealde (Pl) 1. tealdon 2. tealdon 3. tealdon
Sub/Pres (Sg) telle (for all person) (Pl) tellen (for all persons)
Sub/Past (Sg) tealde (for all person) (Pl) tealden (for all persons)
Imp/Pres (Sg) 2 tell (Pl) 2 tellath
Present Participle : tellende, Past Participle : getealde
In current English, the use of the subjunctive mood is very limited. It is used only in unreal conditional clauses and in some that-clauses that follows after certain verbs like 'demand', 'request', etc.. But in Old English, it was used much more frequently, since modal auxiliary verbs were yet to develop in Old English. For example, "He said that she were wise" was used in the sense of "He said that she would be wise" or "He said that she seemed to him wise". Finally, I would add one tip. In current English, past perfect tense is used sometimes in a way as if they were kind of past-past subjunctive, but this usage of perfect tense seems to have developed after the time of Chaucer.
If you would like to know more about this kind of stuff, please visit here, here, and here.
paco