I have written/I wrote–which is which? I always forget
I have written is present perfect.
I wrote is simple past.
Are you asking about the two uses of the Latin perfect? Latin scripsi could be translated into English with either of the above depending on context. A good rule of thumb is that the active perfect defaults to simple past unless there is an obvious reason for present perfect: you answer the door and the person says, veni!); and the passive perfect defaults to present perfective/stative unless there’s an obvious reason for a past focus: mortuus est ‘‘he’s dead’’ but heri mortuus est ‘‘yesterday he died’’.
My fault. I did not ask the question well. What I should have said was that in the context of sequence of tenses rules the present perfect can be used as such or used as the simple past. I see the term historical past used in reference to one of these two forms but I could not remember which one. After posting my question I found an old grammar book and it said that the historical past is the simple past and the perfect is what is referred to as the Perfect Definite. I should also mention that I have sometimes observed that the historical past is called the Perfect Historical. In this case then ‘I wrote’ is the historical past, or at least that is what some people would call it. Personally, I find these terms( historical past, perfect definite, etc.) confusing. I think it muddies the waters in what should be a simple concept to understand such as the sequence of tenses rules. Rather like patients telling their doctor to “Speak English please”.
Your question is still not clear. Are you asking about the sequence of tenses for the subjunctive mood in Latin? The easy way to think about this is to divide all tenses into past/non-past. The perfect is the oddball because it can fall into either category depending on context. When the perfect carries the sense of the English present perfect it functions as a non-past tense.
This dual meaning for the Latin perfect is because of its history: It’s the result of two separate tenses (the aorist and perfect) collapsing into one form. If you study Ancient Greek you’ll find that the Greek aorist (simple past/historic tense) is characterized by vowel changes in the stem or a sigma (s) added to the stem, which is parallel to fac → fec and claud → claus in Latin. The perfect in Ancient Greek is characterized by reduplication of the initial consonant of the stem, which is parallel to tang → tetig in Latin.
For me, knowing why things are the way they are helps me to understand them better. I hope this helps.
calvinist said it well, but you seem to have understood anyway.
To hammer it home:
There are primary (“non-past”) tenses and secondary (aka historic, “past”) tenses; “sequence” is defined thereby.
In Latin (unlike in Greek or English) you have a single form doing double duty. dixi for instance can mean either “I have said” or “I said.”
“I have said” is primary sequence, “I said” is secondary/historic/past.
So verbs in dependent clauses will follow the appropriate sequence-of-tenses rules.
E.g.
diximus ut intelligas (pres.subjunctive, primary sequence) “We have spoken so that you can understand” (“in order than you may understand”)
diximus ut intelligeres (impf.subj., secondary sequence) “We spoke so that you could understand” (“in order that you might understand”)
Etc. etc.
There are various labels applied to the two distinct uses of the “perfect” form. calvinist’s “present perfect” and “simple past” are as good as any. In Greek, where you have two distinct forms, they’re called “perfect” and “aorist” respectively. And you’re quite right, it doesn’t really matter what you call them, so long as you understand the distinction.
I should’ve gave examples, thanks mwh.
Barbarian. intellegeres 
I can’t say anything because I prefer intelligentia, but that assumes a participle intelligens, which assumes a verb intelligere. Oh well, so much for consistency.
Thanks, Calvinist. After reading your reply and looking up what aorist means it is now clear. If it doesn’t open up a can of worms why did the two separate tenses collapse into one? Just natural evolution of the language?
Yes, basically the natural evolution of language. Since it happened before Latin became a written language I don’t think we know why or how it happened exactly, but such a thing is not uncommon in language change. The aorist and perfect both refer to a past event/action, but they differ in aspect. The perfect emphasizes that there is some relevance of that action ‘now’: either the action produced a current state or something similar. There is debate among scholars about the exact meanings of the tenses in Ancient Greek, but there is definitely some semantic overlap between the two. The speakers of Proto-Italic or whatever it would be called at that stage probably felt the distinction wasn’t that important, but I’m definitely not an expert on this.
Now on the other hand, while Greek has a pluperfect and future perfect tense, they are rarely used compared to Latin. So, the distinction between past and past-of-the-past is more explicit in Latin than in Greek. That’s just the way it is with languages, none of them work exactly the same. In fact there are some languages that only have two tenses: past and non-past. Greek also has an optative mood in addition to the subjunctive mood, whereas Latin only has the subjunctive. The optative in Greek was taken over by the subjunctive mood during the Koine period, surviving only in very educated writing or frozen set expressions like the way English “fear not” preserves an archaic construction. Speak not that way, and write not that way, unless you want to sound archaic.
All this to say that such things are very normal in language change. Another thing that can be mentioned is the case system itself. Proto-Indo-European probably had eight cases: Nominative (subject) Accusative (direct object) Genitive (of) Dative (to/for) Ablative (from) Instrumental (by, with) Locative (in, on, at) Vocative (hey you! yeah, I mean you!). There was collapsing here also, and it happened somewhat differently between Greek and Latin: The Latin ablative case is the ablative, instrumental and locative from PIE lumped together. Greek doesn’t have an ablative case, but instead the genitive is the PIE genitive and ablative lumped together and the Greek dative is the PIE dative, instrumental and locative lumped together. I’ve probably said way too much; if I’m confusing you I apologize.
Perhaps a similar example is the observation that the subjunctive seems to be dying out in English speaking countries. I remember in the 60s there was a British rock group who had a song entitled “If she were mine” but the actual lyrics went “if she WAS mine”. I don’t remember having much formal education regarding the subjunctive until I went to college and, studying German, became very acquainted with it. The professor who taught the German course was actually from the linguistics department and I remember him saying that scholars can discern when a change in grammar or pronunciation takes place in a language but there are always debates as to WHY these changes take place. As an example I was thinking of the long vowel shift in English back in the sixteenth century. Anyway, thanks to the posters who answered my original question–I have learned quite a bit.
The English subjunctive has been dying for centuries: I was a bit surprised to see the contrary-to-fact “if … was” in some of Dickens’ lower-class characters in the early- to mid-19th century. Interstingly even Dickens didn’t get it right some of the time (at least according to our usage), using “Bob knocked on the door to see if Bill were home” where we would use “to see if Bill was home” (would this be an indirect question in Latin? The direct question being “is Bill home?”? Something like “ad cernendum utrum Bill domi esset” – I’m bad at composition). That’s subjunctive-ey enough, though, that I can see that that could have been perfectly good English in his day (it’s not like the incorrect “Bill talked to Bob and I” construction). I’m not sure just why the vestiges of the English subjunctive has been so tenacious but it seems to be one of those always-dying-never-dead things (although the present subjunctive – “Bill insisted Bob knock on Joe’s door” – seems to be dying off, which is a bit strange given that it makes an important distinction between “Bill insisted Bob knock” and “Bill insisted Bob knocks” whereas the past “if … were” is more a marker of educated speech than anything else). It’s all very interesting.
Ed: perhaps “insist that Bob knock” isn’t a good example; I was thinking in particular of (very common) politicalese such as “we insist that Iran abides by the agreement” which often trickles down to general speech. I guess the potential for true (as opposed to pedantic) ambiguity is almost nonexistent so the present subjunctive could be seen as superfluous.