Объясните , в чём разница например между: fred is buying a ticket и fred buys a ticket?

b: then where have you been? (present perfect)

a: oh, nowhere. i wanted (past simple) to go to america two years ago,but now i don't want to go there.

b: why?

a: i understood (past simple) that my wish is impossible, so i abandoned(past simple) that idea. 

и 2-я

b: have you heard some interesting news? (present perfect)

a: no.

b: i heard (past simple) that brian flunked (past simple) his exam last week. 

a: why did (past simple)he flunk the exam?

b: he lost (past simple)his homework, so he didn't repeat a topic.(past simple)

mom, dad,  i quit  and come back  later,  because  i was going  to celebrate  the birthday of  a friend  in a cafe.  please  do not worry, i  will come  exactly  at 11 p.m.