For and since are both prepositions of time used with periods that continue up to the present or another reference point. Learners confuse them because both can appear with perfect tenses and both connect time to an ongoing situation. The difference is not the tense itself but whether the focus is on duration or the starting point. For related time expressions, see Prepositional Phrases.

For expresses length of time, so it answers how long something lasts. It is used with durations such as hours, days, months, years, and shorter or longer measured periods. It often appears with the present perfect when an action or state continues, but it can also describe a finished period in the past simple. A broader group of fixed time phrases appears in Idiomatic Prepositions.

Idea	Example
For shows duration.	I have lived here for five years.
For fits measured periods.	She stayed for three days.
For can describe a finished period.	He worked there for ten years before he quit.
For answers how long.	How long have you waited for the bus?

Since expresses a starting point in time, so it answers when something began. It is used with specific dates, years, days, moments, and events, such as a birthday or a move to a new city. It commonly appears with the present perfect to show that the situation began earlier and continues now. In sentences like because is possible, since is a conjunction with a different meaning, not a time preposition.

Idea	Example
Since marks a starting point.	I have known her since 2018.
Since fits specific dates.	They have been closed since Monday.
Since can follow events.	We have been busy since the wedding.
Since shows continuity.	He has lived here since he was a child.

Some sentences can be expressed with either word by changing the form of the time expression. For example, for five years and since 2018 can describe the same span, but one gives the length and the other gives the starting point. The choice depends on whether the speaker thinks in duration or in origin. This contrast is especially clear in During vs While when learners compare time reference ideas.

Idea	Example
Duration can become a start point.	for five years becomes since 2018
For focuses on length.	She has studied for three years.
Since focuses on origin.	She has studied since 2021.
How long questions prefer for.	How long have you worked here for?

Several common phrases are learned as fixed expressions, so they do not always follow the simple length versus starting point rule. For ages means a very long time, for good means permanently, and since forever is an informal way to say from an extremely early time. In spoken English, some dialects drop the perfect tense with since, but standard English usually keeps it.

Word or Phrase	Definition	Example
For ages	It means for a very long time.	I have waited for ages, and he still is not here.
For good	It means permanently.	They moved away for good, and they never returned.
Since forever	It means from a very long time ago.	She has loved music since forever, and it shows.
Nonstandard since	Some informal speech omits the perfect tense.	I know him since school, but standard English prefers the perfect form.

Use for when you say how long something lasts, and use since when you say when it began. For usually pairs with a duration, while since usually pairs with a specific starting point. When a sentence can be rewritten both ways, choose the form that matches the information you want to emphasize. In comparison with During vs While, for and since are less about simultaneous events and more about time length versus time origin.