Consider the following two Spanish sentences:
- Pude abrir la puerta.
1-SG-PAST-PRETERITE-able open SING-FEM-DEF door
- Podía abrir la puerta.
1-SG-PAST-IMPERFECT-able open SING-FEM-DEF door
Both could be translated into English as “I was able to open the door”. The only difference between them is the aspect of the verb poder, “to be able”. In Spanish this aspectual difference has a semantic consequence that does not seem to be directly related to time. Because the first one describes a completed action, it means that I was able to open the door and I in fact did. Grammatically the second one specifies that the action is incomplete, and it means that for some period in the past I had the capacity to open the door (maybe I carried a key in my pocket), but we wouldn’t know if I actually did so until I uttered (1).
Considering just the Spanish here, what strikes me as interesting is that a morphologically aspectual distinction carries irrealis along with it–the salient issue is the possibility versus the certainty that the door was opened, not how the whole business unfolded in time. And in a language with a morphological subjunctive no less. So even when you’re working with a fairly vanilla morpheme, the exact mapping it effects from world to expression can be subtle. Subtle–and still incomplete. There is to my knowledge no ending you can stick on poder that means “At some discrete moment in the past I was able to open the door, but I did not do so.” (Maybe I had the key in my possession for a single day without realizing it and have been filled with regret ever since.) The number of morphemes that will fit in a given slot is always smaller than the number of stories you can invent to confound them. How many stories are there? More than you can count.
Considering a comparison with English, this is a concise example of how different languages use different levels of representation to communicate the same semantics. There is just no compact way in English of making this distinction. The best I can come up with is, “I was able to open the door(, but I never did).” What is a morpheme in Spanish is a whole clause in English. (This is an unusually high inflation rate–a subjunctive Spanish morpheme will typically only blossom into a single English modal verb like “may” or “might”.) It almost makes you think that over here in the English-speaking world we should leave our doors unlocked all the time just to save on the syllables.