If someone had been a teacher from 1975-2000, when he writes about it today, does he say:
I was/have been/had been a teacher from when I (can/could/could have/have) barely remember(ed).
and a slightly different version:
I was/have been/had been a teacher for more than two decades.
I was a teacher a teacher for more than two decades is perfectly good to describe an event or condition from the past that does not continue to be true in the present.
I had been a teacher for more than two decades
works if you want to firmly establish that your being a teacher occurred before some other past event (e.g., I had been a teacher for two decades when I retired).
Among your first group of options, none work, and the word from is problematic. You could say:
I was a teacher so long ago I barely remember it
or
I was a teacher from that day forward.
If it's that particular period you don't remember, you'd need to say
I was a teacher during a period I barely remember
or
at a time I barely remember.
There is actually quite a lot of nuance behind how these words are used, but here are the general rules.
Was indicates a past state that (generally) does not persist to the present. This is the Past Tense.
I was a teacher between 1975 and 2000. But I'm not one anymore.
If you do not specify a period of time, then have been can also be used here. This carries more of a nuance like "I have the experience of having been a teacher".
I have been a teacher. But I'm not one anymore.
With a past time specified, or with other temporal words, have been indicates a state that has persisted from a past time to the present. This is called the Perfect Present Continous Tense.
I have been a teacher since 1975.
Had been indicates a state that persisted from the past until a later point in the past but not necessarily until the present. It may persist to the present, but the author is specifically focusing on a period of time in the past during which something existed in this state. This is the Past Perfect Tense
By the year 2000, I had been a teacher since 1975. (Or, By the year 2000, I had been a teacher for 25 years.)
As for could and can, could is the past test of can (among some other uses). So if the remembering is happening in the present, you use can, if it's happening in the past, you use could.
I have been a teacher since before I can currently remember.
I had been a teacher since before I could at that time remember.
Edit to answer questions asked in a comment:
For more than two decades, IF someone had asked me to desribe myself in one word, it would have been: a teacher
The above sentence is fine with regard to tense, though I would phrase it more like this:
For more than two decades, if someone had asked me to describe myself in one word, my answer would have been "teacher".
The other sentence is also fine with regard to tense:
For more than two decades, had someone asked me to desribe myself in one word, it would have been: a teacher
They both mean the same thing. The second feels perhaps a bit more literary. In conversation, if someone has asked or even just if someone asked are more likely to come up.
Judging from the length of the other answers, most people seem to think this question is more nuanced than I do. Be that as it may, I would simply suggest:
I had been a teacher for as long as I could remember.
After all, if you couldn't remember something in 2000, it goes without saying that you probably still can't remember it fifteen years later.
