2 . My mother has worked at this school for 25 years
( started/began + Ving = has/have + Pii )
3 . The problem has been studied carefully by the scientists
( Bị động hiện tại hoàn thành : have/has + been + Pii +.... )
4 . The girl who is talking to the lecturer has been the best student for 2 years
( Who là mệnh đề quan hệ dùng để thay cho "the girl" để bổ sung ý nghĩa cho chủ ngữ )