1. Use the Present Perfect to say that an action happened at an unspecified time before now. The exact time is not important.
2. When you CANNOT use the Present Perfect with specific time expressions such as: yesterday, one year ago, last week, when I was a child, when I lived in Japan, at that moment, that day, one day, etc.
3. When you CAN use the Present Perfect with unspecific expressions such as: ever, never, once, many times, several times, before, so far, already, yet, etc.
4. . General terms related to case study.
5. 3-4 Topic examples of usage.