What is the difference between since and for?

'Since' is followed by a specific point in time (since Monday, since 2010, since I was a child). 'For' is followed by a period of time (for three days, for two years, for a long time). Both are used with the present perfect tense.

Is it 'since three hours' or 'for three hours'?

The correct phrase is 'for three hours.' 'Three hours' is a duration (a period of time), so use 'for.' Use 'since' only for a specific point in time, like 'since 3 o'clock.'

Can I use since with the past simple?

In some cases yes, but 'since' is most commonly used with the present perfect tense. 'I have lived here since 2015.' Using past simple with 'since' in the main clause is less common and can sound unnatural.

Since vs. For: What's the Difference?

Quick Answer

Since = a specific point in time when something started → "since Monday," "since 2018," "since I was born"
For = a length or duration of time → "for three days," "for two years," "for a long time"

Explanation

SINCE (point in time)

Use since to show when something started:

since Monday
since January
since 2015
since this morning
since she left
since the beginning

Question: Since when? When did it start?

FOR (period of time)

Use for to show how long something lasted:

for three days
for two years
for six months
for a long time
for hours
for ages

Question: For how long? How long did it last?

Tense Note

Both since and for are most commonly used with the present perfect tense when the action continues up to now:

"I have lived here since 2010." (started in 2010, still here now)

"I have lived here for 14 years." (14 years of duration)

Examples

✔I have been waiting since 9 o'clock.

✔I have been waiting for two hours.

✘I have been waiting since two hours.

✘I have been waiting for 9 o'clock.

✔She has worked here since she graduated.

✔She has worked here for five years.

Common Mistake

Incorrect: I haven't seen him since a long time.

Correct: I haven't seen him for a long time.

"A long time" is a duration (period), not a point in time. Always use for with duration expressions.

Quick Comparison Table

Word	Follows	Example
since	a point in time (when)	since Monday / since 2010
for	a period of time (how long)	for three days / for a long time

Practice

Choose the correct word:

"They have been married _____ 20 years." (since / for)

Show answer

Answer: They have been married for 20 years.

"20 years" is a duration, so use for. (Compare: "They have been married since 2004.")

Related Grammar

Starting point versus elapsed time

Since introduces the starting point of a period; for introduces its length. With a situation that continues now, the present perfect is common. For also works with completed past periods: I lived there for two years.

Practise the distinction

We have known each other ___ 2020.
Show answer and explanation
since
2020 identifies the starting point.
We have known each other ___ six years.
Show answer and explanation
for
Six years measures duration.
Is I worked there for a month last summer correct?
Show answer and explanation
Yes.
For can describe a completed period in the past.