1. am – 表示第一人称单数，如“I am”。
2. are – 表示第二人称和第三人称单数，如“You are”。
3. was – 用于过去时态中，表示过去某个时间点的状态，如“He was a teacher”。
4. were – 用于过去时态中，表示过去某个时间点的状态，如“They were students”。
5. have – 表示现在或过去时态中的单数，如“She has a dog”。
6. has – 表示现在时态中的单数，如“He has a car”。
7. haven’t – 表示现在时态中的否定形式，如“I haven’t seen him yet”。
8. haven’t been – 表示过去时态中的否定形式，如“We haven’t been to the beach”。
9. have been – 表示过去时态中的肯定形式，如“I have been to Paris”。
10. have done – 表示过去时态中的完成时态，如“I have done my homework”。
11. have not done – 表示过去时态中的否定完成时态，如“I have not done my homework”。
12. have been doing – 表示过去时态中的进行时态，如“I have been doing my homework”。
13. have not been doing – 表示过去时态中的否定进行时态，如“I have not been doing my homework”。
14. have been doing something – 表示过去时态中的进行时态，如“I have been doing my homework”。
15. have not been doing something – 表示过去时态中的否定进行时态，如“I have not been doing my homework”。
16. have been doing something else – 表示过去时态中的进行时态，如“I have been doing my homework and listening to music”。
17. have not been doing something else – 表示过去时态中的否定进行时态，如“I have not been doing my homework and listening to music”。
18. have been doing something else for a while – 表示过去时态中的进行时态，如“I have been doing my homework for a while”。
19. have not been doing something else for a while – 表示过去时态中的否定进行时态，如“I have not been doing my homework for a while”。
20. have been doing something else for a while and now – 表示过去时态中的进行时态，如“I have been doing my homework for a while and now I am watching TV”。
21. have not been doing something else for a while and now – 表示过去时态中的否定进行时态，如“I have not been doing my homework for a while and now I am watching TV”。
通过学习和练习这些标志性词，你可以更加自信地使用现在进行时，使你的对话更加流畅和自然。