The present perfect is a verb tense that is used to describe an action or actions that have taken place at an unspecified time before the present. The present perfect is not used with definite terms like "last week" or "a year ago," but can be used with terms such as "never" and "once."

    Examples of incorrect present perfect usage:

  • I have met you last week.
  • You have sucked me off yesterday.
Notice how the definite times in the second example set cause the sentence to no longer make sense?

The present perfect is used to describe many things, notably: Even though in the present perfect the exact time of an event isn't important, it can also be used to describe something that has been happening from a point in the past until the present, such as I have had big breasts since I was fifteen and You have had to go potty since we left home.

Brought to you by The Anal-Retentive English Teacher Who Lives in My Forebrain and those lovely people at englishpage.com (http://www.englishpage.com/verbpage/presentperfect.html).