Answer: Ya, which is usually an adverb, is one of those words whose meaning depends almost entirely on the context. Sometimes it doesn't have much of a translatable meaning, becoming a filler word something like pues, adding a slight amount of emotional content to a sentence (although the exact nature of the emotional content may be difficult to determine out of the context).
The most common meanings of ya are "now" and "already," although it can even mean "later." Often, it signifies a slight amount of impatience, although it sometimes can indicate satisfaction or agreement with the person being spoken to. As you might have guessed, it's a word you'll come across more often in informal conversation than you will in formal writing.
Following are some examples of ya in use. Please note that many of the sentences given could be translated or understood differently, again depending on the context in which they're spoken and sometimes the intonation.
- already (the most common meaning when used with a verb in a past tense): Lo he leído ya. I've already read it. El lunes ya lo habré visto. By Monday I will have already seen it.
- now (especially when describing an anticipated action): Ya está aquí. She's here now. Ya salen. They're leaving now.
- now, already (when expressing surprise): ¿Sales ya? You're leaving already? You're leaving now?
- right now: Lo quiero ya. I want it right now. Tienes que estudiar ya. You need to study right now.
- still, any more (especially when used in the negative): Ya no trabaja aquí. He doesn't work here any more.
- to note that a wish has been satisfied: ¡Ya conseguí el trabajo! I got the job! Ya entiendo. Now I understand.
- to indicate frustration: ¡Basta ya! Enough already! ¡Ya está bien! That's plenty! ¡Ya era hora! It's about time!
- to indicate emphasis: ¡Ya lo sé! I already know that! Es difícil, ya verás. It's difficult, you'll see. Ya puedes empezar a estudiar. You had better start studying. Él no comió, que ya es decir. He didn't eat, which is saying something. Ya me gustaría ser inteligente. I'd love to be intelligent.
- later (to indicate something will happen in the indefinite future): Ya ocurriré. It'll happen. Ya lo haré. I'll get it done.
- to express agreement (or, with an ironic tone, to express incredulity): ¡Ya, ya! Oh, sure! Ya, y el papa es luterano. Sure, and the pope is Lutheran. Ya, pero es difícil. Yes, but it's difficult.
- to call attention to something: Ya que no está aquí, podemos salir. Since he's not here, we can leave.
- to offer reassurance: Ya aprobarás el examen. You'll pass the test. Ya verás. You'll see.