Usage of 忘れる, 忘れた, 忘れている, 忘れていた

Question

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 忘れた.

Choices are:

1. 忘れる (forget)
2. 忘れた (forgot)
3. 忘れている (continued state of forgetting)
4. 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 ?

Answer 1

I am pretty sure that wherever you had this question, there was a context because any of these four can be correct depending on the context. If it was asked without context, I will have to say that your source is not reliable for studying Japanese.

1. 僕はこれまで彼をある出来事のために恨んできた。でも、それは三十年前のことだから、もう忘れる。

2. 僕はかつて難しい計算ができた。でも、それは三十年前のことだから、もう忘れた。

3. 僕は難しい計算ができたはずだ。でも、それは三十年前のことだから、もう忘れている。今、試験を受けてみて分かった。

4. そういえば、僕は以前にここに来たことがあった。でも、それは三十年前のことだから、もう忘れていた。

忘れる's inner event is done at an instance. So the use of ている form for this verb means that the completed state sustains. Together with tense, it can mean four things:

• 1). Non-past non-perfective: 'the instantanious event of forgetting happens in non-past'
• 2). Past non-perfective: 'the instantanious event of forgetting happened in past'
• 3). Non-past perfective: 'the state of having forgotten has sustained up till now'
• 4). Past perfective: 'the state of having forgotten has sustained up till past'