Here’s how you can complete the sentences with the correct verb forms:

1. She **has already eaten** five cakes since she was here.
2. When he came back, everything **had already changed**.
3. Almost everyone **had left** for home by the time we arrived.
4. How long **have you learned** English? – for three years.
5. Up to now, he **has not written** any lesson in his notebook.
6. As we **were crossing** the street, we **saw** an accident.
7. Where **were you** last week? – I **went** on holiday to Vung Tau.
8. How many times **have you seen** him since he went to Edinburgh?
9. Are you learning your lesson? – No, I **haven't finished** it. I **will write** a letter.
10. Alexander Fleming who **discovered** penicillin in 1928 **received** the Nobel Prize in 1945.
11. He **did** his homework before he went to the cinema.
12. John **watched** TV at 8 last evening.
13. Birds **build** their nests in summer and **fly** to the South in winter.
14. At this same time tomorrow, we **will be driving** through Pennsylvania.
15. By the next November, I **will have worked** for this company for 6 years.

Feel free to ask if you need further clarification!