Hi ! Was is past simple , you use past simple when you say when the action happened: "I was at home yesterday" You use present perfect tenses when you don't want to say when exactly happened( a year, a day) so you only use expresions like since, already, yet... Or when you want to say that something hasn't been finished yet. If you use the present perfect continuous (I have been...) Normally is for enphasize the duration of the activity : "I have been studying for three hours, I am tired now" If you want to know more about it, visit this web it's very useful http://www.perfect-english-grammar.com/