The document discusses the use of the present perfect tense with "for" and "since" to indicate periods of time. It explains that "since" is used with a point in time to define the starting point of an action, such as "since Monday". It also explains that "for" is used with a duration of time to define how long an action has been occurring, such as "for a week". Examples are provided to illustrate using the present perfect tense with both "for" and "since".