for
prep.prep. used to show who is intended to have or use something, or to explain a reason or purpose. You also use it to talk about a length of time.
prep. indicating the intended recipient, beneficiary, or purpose of an action. Also functions as a temporal marker to denote duration.
I bought a small gift for you.
The local community center provides free classes for adults who want to learn a second language.
The committee debated the proposal for several hours before deciding that the long-term benefits for the environment outweighed the immediate financial costs.
When used to indicate duration, it is followed by a period of time. When used as a conjunction, it is formal and means 'because'.
I have lived here since three yearsI have lived here for three yearsUse 'for' to describe a duration of time; 'since' is only for a specific starting point in the past.