How long you (to wait) for me? i am really very sorry. 4. yesterday i (to meet) a friend of mine whom i (not to see) for a long time. 5. ring me up at eleven o'clock, i (not yet to sleep). 6. you (to be) late for the concert if you (not to take) a taxi. 7. the sun (to set) a long time ago, and it (to begin) to get really cold. 8. when i (to come) home yesterday, my sister already (to re­turn) and (to sit) at the fireplace looking through some old photographs. 9. he (to smoke) three cig­arettes and (to look) through all the books on the shelf, when at last he (to hear) his friend's steps approaching the door. 10. he just (to approach) the door, when she (to enter). 11. he (to write) the composition for three hours and he (to say) he soon (to finish) it as he (to think) over the conclusion now. 12. hardly1 i (to go) out when i (to remem­ber) that i (to forget) to take my umbrella. 13. where is the baby? — the nurse (to put) it to bed. 14. he said he (to work) for a long time with­out achieving good results.

Вот правильные формы глаголов для каждого предложения с объяснением грамматических времен:

• How long have you been waiting for me? I am really very sorry.
  Используется Present Perfect Continuous, так как действие началось в прошлом и продолжалось до момента речи, при этом важна длительность (how long).
• 1. Yesterday I met a friend of mine whom I had not seen for a long time.
     Met — Past Simple для констатации факта в прошлом (yesterday). Had not seen — Past Perfect, так как это действие произошло еще раньше встречи (предпрошедшее).
• 1. Ring me up at eleven o'clock, I shall/will not yet be sleeping.
     Используется Future Continuous (в отрицании), так как речь идет о процессе, который будет (или не будет) происходить в определенный момент в будущем. Также возможен вариант will not have gone to sleep yet (Future Perfect), если акцент на завершении действия.
• 1. You will be late for the concert if you do not take a taxi.
     Это First Conditional (первый тип условных предложений). В главном предложении — Future Simple, в придаточном условия (после if) — Present Simple.
• 1. The sun set a long time ago, and it is beginning (или began) to get really cold.
     Set — Past Simple (факт в прошлом). Is beginning (сейчас становится холодно) или began (начало холодать тогда же).
• 1. When I came home yesterday, my sister had already returned and was sitting at the fireplace looking through some old photographs.
     Came — Past Simple (момент прихода). Had returned — Past Perfect (вернулась раньше, чем я пришел). Was sitting — Past Continuous (процесс в момент моего прихода).
• 1. He had smoked three cigarettes and had looked through all the books on the shelf, when at last he heard his friend's steps approaching the door.
     Had smoked и had looked — Past Perfect, так как эти действия закончились к моменту наступления другого события в прошлом (heard).
• 1. He had just approached (или was just approaching) the door, when she entered.
     Had just approached подчеркивает завершенность к моменту входа, was approaching — процесс в тот момент. Entered — Past Simple.
• 1. He has been writing the composition for three hours and he says he will soon finish it as he is thinking over the conclusion now.
     Has been writing — Present Perfect Continuous (длительность до текущего момента). Says — факт сейчас. Will finish — будущее время. Is thinking — Present Continuous (процесс прямо сейчас).
• 1. Hardly had I gone out when I remembered that I had forgotten to take my umbrella.
     Конструкция Hardly... when требует Past Perfect в первой части (часто с инверсией: Hardly had I gone). Remembered — Past Simple. Had forgotten — Past Perfect (забыл еще до того, как вышел).
• 1. Where is the baby? — The nurse has put it to bed.
     Present Perfect, так как важен результат действия в настоящий момент (ребенок сейчас в кровати).
• 1. He said he had been working for a long time without achieving good results.
     Past Perfect Continuous используется в косвенной речи (после He said) для обозначения длительного действия, которое предшествовало моменту речи.

Для закрепления темы рекомендую изучить таблицу согласования времен в английском языке.