Это упражнение на различие между Present Perfect and Present Perfect Continuous. Если действие уже завершилось и виден результат в настоящем, то это Present Perfect (have/has+третья форма глагола). Если это длительное действие, которое происходило раньше и, возможно, до сих пор продолжается, то это Present Perfect Continuous. Вот и всё)) 1) Don't worry about the letter I have already posted it. (Не переживай о письме, я уже отправил его). 2) Nina has been learning English for 2 years. (Нина учила (и до сих пор учит) английский 2 года. 3) She has been playing tennis since she was eight. 4) The birds have already flown to the South. 5) The train has just come. 6) She has been teaching at this school for twenty years.
3) I have been reading Tolstoy stories in the park for two hours. 4) I have read three stories. 5) Granny has been waiting in the living room since morning. 6) She has knit a sweater for me. 7) My sister has been skating since childhood. 8) She has taken part in many competitions.
1- had returned
2- had finished
3- had seen
4- had cook
5- had done
6- had bought
7- hadn't washed
8- had gone
9- had left
10- hadn't done
11- hadn't came yet
12- had lost