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My students always ask me the same question: “Teacher, what’s the difference between ‘by’ and ‘until’?” And every time, I see the same moment of confusion when I explain it. The words look similar, both relate to deadlines and time, and in some sentences you could use either one — but they don’t mean the same thing. If you pick the wrong one, your sentence shifts meaning in ways that matter.
You’ll breaks down the essential difference, shows you when each word works, and gives you a simple test you can use whenever you’re unsure. By the end, you’ll know instantly whether to use “by” or “until” — and you’ll understand why the choice matters.
Key Takeaways
- BY = deadline / completion point. “By Friday” means it will be done sometime before or at Friday — don’t wait past Friday.
- UNTIL = duration / continuation. “Until Friday” means something keeps going up to (and stopping at) Friday.
- Test: Replace BY with “no later than” and UNTIL with “continuing up to.” If your sentence makes sense, you’ve chosen correctly.
- BY is about starting/finishing a task. UNTIL is about how long something continues or doesn’t happen.
- Same time reference, different meanings: “Be in bed by 10” (get in bed anytime before 10) vs. “Stay in bed until 10” (remain in bed, leave at 10).
When to Use “BY”
Use “by” when you’re setting a deadline or final point — the latest moment when something must happen or be completed.
Structure with BY
- By tomorrow
- By next week
- By six o’clock
- By Wednesday
- By summer
- By August 31st
- By the end of the month
Real Examples with BY
Example 1: “I’ll be back by 11 o’clock.”
The deadline is 11 o’clock. You promise to return anytime before 11 — maybe 10:30, maybe 10:59, but definitely not after 11.
Example 2: “You have to finish this assignment by August 31st.”
August 31st is the last day you can finish. You could finish it earlier, but it must be done by that date.
Example 3: “Will the details be available by December?”
This asks whether the information will be ready no later than December — meaning ready sometime in December or earlier.
Example 4: “She has to hand in her project by Friday.”
Friday is the deadline. She could turn it in Thursday, but it must be submitted no later than Friday.
Example 5: “Don’t worry, you will get your books by Thursday.”
You will receive them sometime before Thursday or on Thursday at the latest.
Tip: Think of “by” as a line you cannot cross. Once you reach the “by” time, it’s too late to start — the task should already be finished.
When to Use “UNTIL”
Use “until” when you’re describing duration — how long something continues from now until a future point, or how long something doesn’t happen.
Structure with UNTIL
- Until tomorrow
- Until next week
- Until six o’clock
- Until Wednesday
- Until summer
- Until August 31st
- Until the end of the month
Real Examples with UNTIL
Example 1: “Alan came yesterday. He will stay with us until the weekend.”
Alan is here now, and he will continue to be here through the weekend. At the weekend, he leaves. The staying happens up to the weekend.
Example 2: “They lived in a small house until September 2003.”
They were in that house, and the living continued until September 2003. In September 2003, they left or moved. The duration of living = until September.
Example 3: “I will be away until Wednesday.”
I am away now, and I will be away throughout the days up to Wednesday. On Wednesday, I return.
Example 4: “She won’t have any free time until Wednesday.”
She is busy now, and she stays busy all the way up to Wednesday. Starting Wednesday, she has free time.
Example 5: “Yesterday he stayed in bed until 12:00.”
He was in bed, and remained in bed until noon. At noon (or after), he got up.
Remember: “Until” describes continuation of a state or action. Something is happening (or NOT happening) and will continue to do so until the time you mention.
The Critical Difference: One Sentence, Two Meanings
This pair shows exactly why BY and UNTIL matter:
Situation: A person has a high fever and needs to rest.
Using UNTIL: “He needs to be in bed until 10 o’clock.”
Meaning: He is already in bed. He must stay there, continuing to rest, up to 10 o’clock. At 10 o’clock, he can get up.
Using BY: “He needs to be in bed by 10 o’clock.”
Meaning: He is not in bed yet. But for his health, he should get into bed before 10 o’clock. 10 o’clock is his deadline to be lying down.
The time reference is the same (10 o’clock), but the meanings are opposite:
- UNTIL 10 = he’s already in bed; stay there until 10.
- BY 10 = he’s not yet in bed; get there before 10.
Formula:
BY = “no later than” / “deadline for completion”
UNTIL = “continuing up to” / “duration endpoint”
BY and UNTIL in Negative Sentences
“Until” is commonly used in negative sentences:
Example 1: “Details will not be available until January.”
Meaning: You cannot get the details before January. January is the earliest date they will be ready.
Example 2: “He won’t arrive until Tuesday.”
Meaning: He is not here now, and he still won’t be here before Tuesday. Tuesday is when he finally arrives.
Example 3: “We won’t start the meeting until everyone is present.”
Meaning: The meeting is delayed. We will wait (not start) until everyone shows up.
You can use “by” in negatives too, but it’s less common:
Example: “Don’t call me by 9 a.m.” = “Don’t call me before 9 a.m.; wait until after 9.”
Side-by-Side Comparison Table
| Feature | BY | UNTIL |
|---|---|---|
| Core function | Deadline; point of completion | Duration; how long something continues |
| What it answers | “When must this be done?” | “How long will this last?” |
| Synonym | “No later than” | “Continuing up to” |
| Example | Submit by Friday | Stay until Friday |
| Tone | You must act before this time | Something continues/happens up to this time |
| Past tense | Rare; usually present/future | Common; “We waited until noon” |
| In negatives | Unusual | Very common; “not… until” |
Common Mistakes with BY and UNTIL
✗ Incorrect: “I will finish this project until Monday.”
✓ Correct: “I will finish this project by Monday.”
Why: “Finish” is a completion action, so you need a deadline (BY), not a duration (UNTIL).
✗ Incorrect: “Can you wait by 3 o’clock?”
✓ Correct: “Can you wait until 3 o’clock?”
Why: “Wait” is a state of continuation, so UNTIL is correct. BY is for deadline actions.
✗ Incorrect: “I need this report until the end of the week.”
✓ Correct: “I need this report by the end of the week.”
Why: “Need” implies a deadline for completion. You’re setting a deadline, not describing duration.
✗ Incorrect: “The store is open by 9 p.m.”
✓ Correct: “The store is open until 9 p.m.”
Why: The store is operating (a continuing state) up to 9 p.m. UNTIL describes this duration.
Dialogue: BY and UNTIL in Real Conversation
Manager: “When will you have the report ready?”
Employee: “I’ll finish it by Friday at 5 p.m.”
Manager: “Perfect. And will you be available for the presentation?”
Employee: “I’ll be in the office from Monday until Wednesday afternoon, then I’m taking leave.”
Manager: “That works. We’ll schedule the presentation by Wednesday before you leave.”
Employee: “Great. I can stay until 4 p.m. that day.”
Quick Practice Quiz
Quick Quiz
- Choose the correct word: “I’ll have the dishes cleaned _____ dinner.” (a) by (b) until
- Which sentence is correct? (a) “Wait until I get back” or (b) “Wait by I get back”?
- “The office is open until 6 p.m.” means: (a) it closes at 6 p.m. (b) it opens at 6 p.m.?
- Fill in: “Please submit your homework _____ next Monday.” (a) by (b) until
- “I won’t be home until 8 p.m.” means: (a) I’m home now and leaving at 8 (b) I’m not home yet and arriving at 8?
Answers: 1. (a) · 2. (a) · 3. (a) · 4. (a) · 5. (b)
Time Expressions with BY and UNTIL
BY — Common Time Phrases
- By tomorrow morning
- By the end of this month
- By next Friday
- By 3 o’clock
- By the time we arrive
- By summer break
- By the deadline
UNTIL — Common Time Phrases
- Until next week
- Until we hear from you
- Until the end of the show
- Until further notice
- Until tomorrow afternoon
- Until the last minute
- Until you graduate
Related Articles
- ↑ Master Pillar: English Grammar
- English Expressions & Phrases — Complete Guide
- ↑ Back to pillar: English Expressions
Frequently Asked Questions
Can I use “by” and “until” interchangeably in the same sentence?
No. “By” and “until” shift the meaning of a sentence. “By Friday” and “until Friday” are not the same. Always check whether you’re setting a deadline (by) or describing duration (until) before choosing.
In negative sentences, why is “until” so common?
Negative sentences with “until” focus on duration of non-action: “I won’t eat until dinner” means I’m not eating now and will continue not eating until dinner. “By” would be awkward here because you’re not describing a deadline for eating — you’re describing how long you’ll wait.
Can I use “by” to talk about the past?
Yes, but it’s less common. “By the time I arrived, he had left” uses “by” correctly to mark a point in the past. However, “until” is far more common in past-tense narratives: “He waited until I arrived.”
What’s a simple memory trick to remember the difference?
Think: BY = “no later than” (you set a deadline). UNTIL = “continuing up to” (something goes on until that point). If you can substitute these phrases, you’ve chosen the right preposition.
Is “till” the same as “until”?
Yes. “Till” is an informal, shortened version of “until.” They mean exactly the same thing. “Until” is more formal and preferred in written English, but “till” is perfectly acceptable in speech and casual writing.
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