I really don't think this is the case. First of all, should as a past tense of shall seems like a pretty unhelpful simplification. It only acts that way under quite specific circumstances. In general, shall and should mean very different things. Should has some subjunctive uses, but they are very limited as well. Ex: A construction with the modal should is frequently used as an alternative to the simple present subjunctive, e.g., It is important that he should be cured. -Wikipedia It's definitely an interesting thought, but think that it's a massive oversimplification and I don't think it's helpful either. Probably better to just look at more sentences and learn specific uses. You can read more about English subjunctive here: https://en.wikipedia.org/wiki/English_subjunctive