For vs. Since: What's the Difference?
Quick Answer
"For" is used with a length or duration of time — how long something has lasted (for three years, for a long time). "Since" is used with a starting point in time — when something began (since 2020, since Monday, since I moved here). Both are common with perfect tenses, but they answer different questions: "for" answers "how long?" and "since" answers "from when?"
Difference at a Glance
| Feature | For | Since |
|---|---|---|
| Answers the question | How long? | From when? / Starting when? |
| Followed by | A duration (three years, a week, ages) | A point in time (2020, Monday, I was young) |
| Example | for two hours | since 3 o'clock |
| Common tense pairing | Present perfect / past perfect | Present perfect / past perfect |
Definitions
For
Used with a period or duration of time — a length of time, not a specific starting moment.
for + duration
"for three years"
"for a long time"
"for two weeks"
Since
Used with a specific starting point in time — a date, event, or moment.
since + point in time
"since 2020"
"since I graduated"
"since last Monday"
Grammar Rule
Key Rule: Use "for" before a length of time (an amount, like "three years" or "a while"). Use "since" before a fixed point in time (a date, day, event, or clause describing when something started), typically with the present perfect or past perfect tense.
Quick test: if the phrase answers "how much time?", use "for". If it answers "starting from when?", use "since".
Common Mistakes
Incorrect: I have lived here since three years.
Correct: I have lived here for three years.
"three years" is a duration, so it needs "for", not "since".
Incorrect: She has worked here for 2019.
Correct: She has worked here since 2019.
"2019" is a fixed point in time, so it needs "since", not "for".
Incorrect: We haven't spoken since a long time.
Correct: We haven't spoken for a long time.
"a long time" is a duration, so it needs "for".
More Correct Examples
Mini Quiz
1. Choose the correct word: "We have known each other _____ high school."
2. Choose the correct word: "We have known each other _____ ten years."
3. Fix the sentence: "He has been unemployed since six months."
Common Learner Questions
Can "since" be followed by a clause instead of a date?
Yes. "Since" can introduce a time clause, as in "since I moved to this city" or "since she started her new job." The clause describes the starting point, just like a date would.
Do "for" and "since" only work with the present perfect tense?
No, though that is their most common use. They also appear with the past perfect ("She had lived there for ten years before she moved") and, less often, other tenses when talking about duration or a starting point.
Is it correct to say "for a long time ago"?
No. "Ago" already marks a point in the past counted back from now (three years ago), so it should not be combined with "for". Use either "for three years" or "three years ago", not both together.