This document provides guidance on using "for" and "since" with the present perfect tense. "For" is used with a period of time, such as "We have studied for two hours." "Since" is used to refer to a specific point in time, like "We have studied since ten o'clock." It also notes that the present perfect can be used with "never" to refer to something that hasn't happened, as in "My mother has never been to the U.S."