Since vs For: Duration and Starting Points

Since and for are the two words English uses to talk about how long something has been going on, and mixing them up is one of the most common errors at intermediate level. The rule itself is short: for measures a length of time, and since marks the moment something began. Once you can tell a duration from a starting point, the choice is almost automatic.

For measures how long

Use for with a period of time, the answer to how long.

• I have lived here for five years.
• We have been waiting for twenty minutes.
• She has known him for a long time.

If you can put the phrase after how long (how long? for five years), then for is correct. It is about the length of the stretch, not when it began.

Since marks when it started

Use since with a point in time, the answer to since when.

• I have lived here since 2019.
• We have been waiting since ten o'clock.
• She has known him since university.

Since points to the start and implies the situation has continued up to now. That is why it lives so naturally with the present perfect, and with its ongoing form the present perfect continuous (I have been waiting since ten): the action began at that point and reaches the present. Using since and for accurately with the present perfect is a recognised B1 milestone on the CEFR scale, so a free English level test is a good way to check this structure is solid.

The other since: because

Since has a second, separate job: it can mean because.

• Since you are already here, let us begin.
• Since it was raining, we stayed in.

Here since introduces a reason, not a time. Context makes it obvious, so this rarely causes confusion, but it is worth knowing that one word carries both meanings.

Quick guide

Ask what follows the word. A length of time (two hours, ten years, a while)? Use for. A starting point (Monday, 2010, I was born)? Use since, usually with the present perfect. And if what follows is a reason rather than a time, since means because. One sentence holds the time uses: "I have worked here for three years, since I left university." For another way English handles time and the past, see used to vs would.