The shortest adverb in Spanish, the word ya derives from the Latin iam, which is also the origin of the Portuguese já, French déjà, and Italian già. Iam also originated another Spanish adverb: jamás ("never," iam + magis).
But the use of the word ya in Spanish has evolved beyond its function as an adverb of time meaning “already.” Nowadays, ya can be used as a conjunction, an interjection, a different type of adverb, or even as part of idiomatic phrases. It's actually a very popular word! Let's see a few examples.
First, let's see an example where ya simply means "already":
Ya tenemos listo aquí nuestro pollo.
We already have our chicken ready here.
Caption 17, [Bears in the Kitchen] Osos en la cocina - Pollo asiáticoPlay Caption
One interesting usage of the word ya is as a conjunción distributiva (the equivalent in English are correlative conjunctions). The classic way to do so is by repeating the word ya before each option in a given list of items, for example: Ya con alegría, ya con tristeza (whether with happiness, whether with sorrow). However, this is a little bit too poetic for everyday speech, so you would find that people substitute the second ya with a more common conjunction, the disjunctive o (or). For example:
...ya sea en ayunas o luego de haber comido algo.
...whether fasting or after having eaten something.
Caption 12, Los médicos explican - La diabetesPlay Caption
You may have noticed the use of the verb sea, subjunctive of the verb ser (to be). This combination is very common, so you may want to add ya sea (whether it be) as a single expression in your vocabulary. Take note that sea can be omitted too in Spanish, so you can say: ya en ayunas, o luego de haber comido.
Another common use of ya is when it's combined with the conditional si (if). It may translate as "already" in some cases:
Si ya estás instalado en Barcelona...
If you are already settled in Barcelona...
Caption 63, Blanca - Cómo moverse en BarcelonaPlay Caption
Or as "now":
Si ya no nos queda nada porque luchar
If now there is nothing left for us to fight for
Or as "anymore":
Si ya no me quieres...
If you don't love me anymore...
Ya meaning "not anymore" is always accompanied by negation, of course. Ya no (“no more,” sometimes also translated as “enough”) is a very common expression too, definitely worth adding to your lexicon.
Los medicamentos caducados o
Expired medications or
que ya no vayas a necesitar...
[ones] that you are not going to need anymore...
Caption 69, 3R - Campaña de reciclajePlay Caption
Here's another one, with a little extra (the idiom hacer caso means "to pay attention"):
Lo que pasa es que ya no le hago caso.
The thing is that I don't pay attention to him anymore.
Caption 50, Guillermina y Candelario - El parque de diversionesPlay Caption