What Each Word Actually Does

For, since, and ago all help you talk about time, but they do different jobs. Mixing them up is one of the most common mistakes at this level, so let's sort them out clearly.

For tells you how long something lasts. It answers the question "How long?" and is followed by a duration — a length of time.

✅ I've lived in Berlin for three years.

Since tells you when something started. It answers "From what point?" and is followed by a specific moment in the past — a date, a year, a time, or an event.

✅ I've lived in Berlin since 2021.

Ago tells you how far back something happened. It goes after the time expression and is used with the past simple, not the present perfect.

✅ I moved to Berlin three years ago.

Notice that all three sentences above describe roughly the same situation, but from different angles. That's the key: they aren't interchangeable — each one requires a different kind of time reference and sometimes a different tense.

For vs. Since — The Mistake Nearly Everyone Makes

The single most frequent error is using since with a duration instead of a point in time:

❌ I've worked here since five years. ✅ I've worked here for five years.

"Five years" is a duration (how long), so it needs for. Compare:

✅ I've worked here since 2019.

Here, 2019 is a point in time (when it started), so since is correct.

A reliable test: can you put "from" in front of the time expression and have it make sense? From 2019 — yes, use since. From five years — no, use for.

Both for and since are common with the present perfect (have/has + past participle) when the situation is still true now. You can also use for with other tenses — I waited for twenty minutes (past simple), I'll be away for a week (future) — but since almost always pairs with a perfect tense.

Ago — Position and Tense

Ago behaves differently from the other two. It comes after the time expression, not before it:

❌ I started this course ago two months. ✅ I started this course two months ago.

And because ago points to a finished moment in the past, it takes the past simple:

❌ I have graduated a year ago. ✅ I graduated a year ago.

Using the present perfect with ago is a very natural mistake — many learners feel that "a year ago" still connects to now. It doesn't, grammatically. Ago marks the action as completed and located in past time, so the past simple is required.

Quick-Reference Contrasts

Sometimes seeing all three side by side in the same context locks the pattern in:

✅ She has known him for ten years. ✅ She has known him since secondary school. ✅ She met him ten years ago.

All three are correct and describe the same relationship, but the grammar around each word is different. Change any piece — swap the tense, move ago before the number, put a duration after since — and the sentence breaks.

One last edge case worth knowing: for can sometimes be dropped in informal speech (I've been waiting twenty minutes), but since and ago can never be dropped. And don't confuse since the time word with since meaning "because" — context usually makes it obvious, but in grammar exercises, always check whether the sentence is about a reason or a starting point.

Keep these three distinctions sharp — duration, starting point, finished past — and the right word will follow naturally.