Consider this sentence:
それは三十年前のことだから、もう[fill in the blank]。
The answer given by the workbook is
- 忘れる (forget)
- 忘れた (forgot)
- 忘れている (continued state of forgetting)
- 忘れていた (had forgotten, currently may or may not be forgotten)
My thought process:
Choice 1 is ruled out because it is present/future tense, but the question describes something in the past.
Choice 3 cannot make sense to me. If one is able to talk about something that one forgets, the person is currently not forgetting it. (I do not see how this can make sense in the first person) (Unless it's a dynamic state of memory where the person keeps forgetting and recalling bits and pieces?)
Choice 4 was my choice. I don't understand why 2 is correct while 4 is not. I thought 4 to be correct because he had forgotten about the issue, but is currently remembering it while making the statement.
(Question) Why is choice 2 correct and 4 wrong? And under what circumstances do we use 3 ?