<em>For</em> is followed by a length of time: 'We've lived here for three years.' <em>Since</em> is followed by the point at which something began: 'We've lived here since 2021.' Both describe the same stretch — one measures it, the other marks where it started.
The core difference
Ask yourself what comes after the word. For takes a duration: two hours, six months, a decade, ages. Since takes a moment: Tuesday, 2019, breakfast, the moment she left.
The same fact can be expressed either way. If you started a job in January and it is now July, you have been there since January and for six months.
The test
Ask 'how long?' or 'starting when?' If your answer is a quantity of time, use for. If it is a date, a clock time, or an event, use since.
A supporting check: since almost always sits in a perfect tense. 'I have known her since university' is right; 'I know her since university' is not. For is more flexible and works with simple tenses as well: 'We waited for an hour.'
Examples side by side
- He's been off work for two weeks. / He's been off work since the accident.
- The shop has stood empty for years. / The shop has stood empty since the flood.
- She practised for an hour. / She's been practising since six o'clock.
The tense trap
This is the mistake that actually causes trouble for people learning English. Since requires the present perfect or past perfect, because it describes a period running up to now (or up to a past reference point):
- Wrong: I am here since Monday. Right: I have been here since Monday.
- Wrong: She works here since 2020. Right: She has worked here since 2020.
For does not force this, but the meaning changes with the tense. 'I lived in Leeds for five years' says the period is over. 'I have lived in Leeds for five years' says I still live there.
Other jobs these words do
For has many uses beyond time — purpose ('for the meeting'), recipients ('for you'), price ('for £40'). None of those compete with since.
Since also works as a conjunction meaning 'because', which is a separate matter entirely: 'Since you're here, help me lift this.' Keep the two senses apart when you write, because a reader meeting since at the start of a sentence has to decide which one you meant.
Note also that for can be dropped in some sentences without loss: 'We waited an hour' is as natural as 'We waited for an hour'. Since can never be dropped.
Frequently Asked Questions
Is it 'since three years' or 'for three years'?
'For three years.' 'Three years' is a length of time, so it needs 'for'. 'Since' would need a starting point, as in 'since three years ago'.
Is it 'I work here since 2020' or 'I have worked here since 2020'?
'I have worked here since 2020.' 'Since' requires a perfect tense because it describes a period continuing up to now.
Can I say 'since last week'?
Yes — 'last week' names a point you are counting from, so 'I've been ill since last week' is correct. Compare 'for a week', which measures the length instead.