English speakers learning Spanish have a hard time understanding the similarities and differences between ya, todavía, and aún (or aun). They don't perfectly match up with the similar English words "already," "yet," and "still." What concept exactly does each of the Spanish words express? Is there a relatively easy way for a language learner to understand the differences, or is it a matter of memorizing which word is used in which type of phrase?
In English dialects, there are subtle differences between the meanings of one of these three words. If you go back some seventy years, the phrase "Is he yet reading?" would have been understood differently in Ohio and in New England.
In New England, this phrase would have been understood as a stilted way of saying, "Is he reading yet?", something one would say of a child.
In Ohio, it meant, "Is he still reading?", something one would say of an old man.
The matchup for ya, aún, and todavía is close but not exact. You shouldn'r expcet it to be exact. With prepositions, the mismatches are even more evident.
Ya: When a positive finished action is spoken. "I have already done the homework" would be "Ya he hecho los deberes".
Todavía and aún. I would say they are perfect synonyms, as everyone outlined in this post. But I want to note something. If I said 'todavía', I would be more likely to do the action after saying that. Also, it is expected to me to do it and I kind of deceive someone if I don't do it. If I said 'aún', which would be slightly more vague, I am just informing, not making up any excuse.
Examples: Todavía no he arreglado el cuarto. Aún no he arreglado el cuarto. First one would be said when slightly deceive someone (sorry, I haven't tide up my room yet), second one I'm just informing or complaining (I haven't tide up my room so I cannot find anything!).
It's just my Spanish point of view. It also might depend of regions.
The english equivalents would be:
Ya = Already
Todavía = Aún = Yet = Still
Aún also means even (With a meaning of time):
Aún cuando había pagado, el señor no me dejó ir = Even when I had paid, the man didn't let me go.
Aun is written without accent when it doesn't mean still (Todavía):
No tengo tanto dinero, ¡ni aun la mitad! = I don't have that much money, not even half of it!