When you use the Present Perfect with for or since, you are describing a situation that began at some point in the past and has continued up to the present moment. The situation is not over — it is still happening right now. This is what makes it different from the Past Simple: the Past Simple describes something finished; the Present Perfect with for or since describes something that bridges the past and the present.
Think of it as a bridge. On one end is a moment in the past — when the situation began. On the other end is now — today, this moment. The situation has been crossing that bridge the whole time and is still on it. For and since are the two ways of describing this bridge.
For describes how long the situation has lasted. It tells you the length of the bridge — the amount of time that has passed. You use for with a period of time:
Since describes when the situation began. It tells you where the bridge starts — the specific moment in the past when everything began. You use since with a point in time:
| FOR | SINCE |
|---|---|
| Describes a duration — a length of time | Describes a starting point — a moment in time |
| Answers: How long? | Answers: From when? |
| Used with periods: two hours, six months, a long time, ages | Used with points: 2018, last Monday, I was a child, the accident |
| I have lived here for ten years. | I have lived here since 2015. |
The question How long...? is the natural question for situations described with for and since. It is always answered with the Present Perfect:
✗ I have lived here since ten years. · She has worked here for 2018. · We have known each other since a long time.
✓ I have lived here for ten years. (ten years = duration → FOR)
✓ She has worked here since 2018. (2018 = starting point → SINCE)
✓ We have known each other for a long time. (a long time = duration → FOR)
Test it with a simple question: Is this a length of time or a point in time? If you can say "two hours / six months / a long time" — use for. If you can say "2018 / last Monday / when I was a child" — use since. Numbers of years, months, hours, and days are always durations → for. Years as calendar points (2018, 1995) are always starting points → since.
✗ I live here for ten years. · She works here since 2018. · How long do you know her?
✓ I have lived here for ten years. · She has worked here since 2018. · How long have you known her?
This is the most common error, especially for speakers of Romance languages (French, Spanish, Italian, Portuguese) where the present tense is used for ongoing situations. In English, you must use the Present Perfect — not the Present Simple — when describing a situation that started in the past and is still continuing. The Present Simple would imply the situation is simply a permanent fact with no time reference.
✗ I lived here for ten years. (implies I no longer live here) · She didn't eat meat since she was a teenager.
✓ I have lived here for ten years. (I still live here now) · She hasn't eaten meat since she was a teenager.
The Past Simple with for means the situation is now over: I lived there for ten years = I no longer live there. If the situation is still true now — you are still living there, still working there, still not speaking — use the Present Perfect. The tense you choose signals whether the situation continues into the present or has ended.
✗ I have known her since we have been at school. · He has worked here since he has joined the company.
✓ I have known her since we were at school. · He has worked here since he joined the company.
When since introduces a clause (a subject + verb), the verb in the since clause is in the Past Simple — not the Present Perfect. The main clause uses the Present Perfect; the since clause uses the Past Simple to mark the starting point. Think: He has worked here (PP — main clause, ongoing) since he joined (PS — since clause, the starting event).