"By" indicates a deadline (no later than); "until" indicates continuous duration up to a point.
Time preposition. Indicates a deadline: the action must happen at some point before or at that moment. Does not imply continuity. Synonyms: no later than, at the latest. In Spanish: "para las..." (by a time) or "para el..." (by a date).
Please send me the report by Friday.
Por favor enviame el informe para el viernes (a mas tardar).
The package should arrive by next Tuesday.
El paquete deberia llegar para el martes que viene.
You need to finish this by noon.
Necesitas terminar esto para el mediodia.
Time preposition. Indicates that an action or state continues uninterrupted up to a point in time. Implies continuity from start to that moment. In Spanish: "hasta."
She worked until midnight to finish the project.
Ella trabajo hasta la medianoche para terminar el proyecto.
The store is open until nine at night.
La tienda esta abierta hasta las nueve de la noche.
Wait until I get back before starting dinner.
Espera hasta que yo regrese antes de empezar la cena.
Common mistake
Classic error: "I will work until Friday to finish this" could be correct (you will keep working continuously until Friday), but if you mean Friday is the deadline, use "by Friday." The reverse: "The store is open by 9pm." Wrong for saying it closes at 9. Correct: "The store is open until 9pm." (it operates continuously up to 9). "By" does not imply continuity; "until" does.
Trick to remember it
"By" = deadline. Is there a time limit? = by. Think of "by [date/time]" as a limit: "terminalo para el lunes" = "finish it by Monday." "Until" = continuity. Is the action ongoing up to that point? = until. "Trabaja hasta las 5" = "work until 5." Key question: is the action continuous or is it an event that must happen before a date?
Quick rule
By = no later than (deadline). Until = up to (continuity).
Did you know?
"Till" is an informal synonym of "until" and is completely correct: "wait till I get back." Very common in colloquial American English. And "by now" (already, at this point) is an idiomatic expression: "You should know this by now." No confusion with "until" there. Also, "by then" = "by that time": "We should be done by then." A useful phrase for expressing expectations about a future moment.
Exercise 1 · Pick the right one
The library is open _____ ten o'clock tonight. You can study there.
Exercise 2 · Pick the right one
You must submit your application _____ the fifteenth of this month.
Exercise 3 · Form the word
By = no later than (deadline). Until = up to (continuity).
Classic error: "I will work until Friday to finish this" could be correct (you will keep working continuously until Friday), but if you mean Friday is the deadline, use "by Friday." The reverse: "The store is open by 9pm." Wrong for saying it closes at 9. Correct: "The store is open until 9pm." (it operates continuously up to 9). "By" does not imply continuity; "until" does.
"By" = deadline. Is there a time limit? = by. Think of "by [date/time]" as a limit: "terminalo para el lunes" = "finish it by Monday." "Until" = continuity. Is the action ongoing up to that point? = until. "Trabaja hasta las 5" = "work until 5." Key question: is the action continuous or is it an event that must happen before a date?
Bryan López
Founder of SpeakUP Academy