vs
QUICK ANSWER
"By now" is a phrase which is often translated as "ya", and "now" is an adverb which is often translated as "ahora". Learn more about the difference between "by now" and "now" below.
by now( 
bay
nau
)A phrase is a group of words commonly used together (e.g., once upon a time).
1. (general)
c. a estas horas 
He probably knows by now that he can't fire judges he disagrees with.Probablemente sabe a estas horas que no puede despedir a los jueces con los que no está de acuerdo.
now(
nau
)An adverb is a word that describes a verb, an adjective, or other adverbs (e.g., to run quickly, very tired).
2. (nowadays)
a. actualmente 
Now things are done much differently.Actualmente las cosas se hacen de manera muy diferente.
An adjective is a word that describes a noun (e.g., the big dog).
A conjunction is a word that connects words, phrases, clauses, or sentences (e.g., The cat and the dog slept.).
a. ahora que 
Now that you have it, what are you going to do about it?Ahora que lo tienes, ¿qué vas a hacer al respecto?
A noun is a word referring to a person, animal, place, thing, feeling, or idea (e.g., man, dog, house).