We have lived in this house for twenty years. = Twenty years ago, we started living in this house. We still live in this house now.

He has climbed Mount Everest three times. = He climbed Mount Everest in 1988, 1999, and 2012. He might climb it again.

Sara has not completed the project yet. (negative form) = Sara started the project on Monday. Today is Friday, and the project is still not done.


Present Perfect Continuous

Use: actions started in the past and continue now. Usually the length of time is specified in the sentence: since Tuesday, all year long, for more than an hour.

Form: has/have been VERB+ing
Negative Form: has/have not been VERB+ing

They have been making the same mistakes for years. (present perfect continuous)

Ever since her father died, she has been working on a cure. (present perfect continuous)

She has not been feeling well lately. (negative form)

David is writing an article for this month's journal. (present continuous)