English Tenses

I could help her, and I did, or I could have helped her, but I didn't.

Thanks very much! It’s much clearer now; however, “can” in the Past Indefinite Tense is “could”, which makes the whole situation a bit confusing, like in “I could help her, and I did.” (no subjunctive). If I’m correct in my understanding, “I could help her, but I didn’t” is wrong because you automatically go into the subjunctive mood if it is negative. Right?

Yes, because the subjunctive is for counterfactuals.

Thanks very much, Centrox!