Definition:
1) The present perfect is used to say what has happened recently and has an effect on the present moment. We often use ‘just’, ‘yet’ and ‘already’ to express the relationship to the present moment. For example, They haven't had lunch yet.
2) When speaking about an experience that has happened at an unspecified point in time before the present moment use the present perfect. They've seen the film twice.
3) When an action begins in the past and continues into the present moment we use the present perfect. This use of the present perfect expresses duration over time FROM the past moment TO the present moment. For example, Alice has lived in New York since 1997.
