Hello. I - Simple past is used in situations 1 - finished on defined past and 2- habitual actions finished on the past. About your doubt - "I graduated" (means "I finished my course"). II - Present perfect is used to express indefinite actions on the past: 2a- "I have graduated" means "I am studying for graduate since 2016", for example. 2b- "Mr. Hook has studied the moon for 30 years". "Mr. Hook has studied the moon since 1960". I hope that helps!

thanks a lot I guess I got it

Past simple is used for something that happened in the past and is no longer happening. Present perfect is used for something that started in the past, but is still continuing in the present (which is why it is 'present' perfect). Both 'I graduated' and 'I have graduated' are correct, and here's why: It is correct to say 'I graduated last Tuesday', but you can't say 'I have graduated last Tuesday'. That's because the first example is talking about the physical act of graduating. It is something you did (last Tuesday). The reason why we can use present perfect ('I have graduated') is because even though the physical act was in the past, you still have a certificate that says that you graduated. For the rest of your life you will hold the qualification that you gained when you graduated. Because of this, the 'experience' is still continuing in the present. This is the same reason we can say 'I have been to England' instead of just 'I went to England'. The sentence implies that you gained an experience while in England, and that experience is still with you. I'm sorry for my complicated answer, I hope it's understandable for you. Past simple and present perfect can be quite confusing.

Using simple past tense when you have specific time in the past. Example: I graduated a month ago. Using present perfect when you don't have spesific time, but you're sure you have done it. Example: I've graduated from Junior high school. Hope it will help.