All sentences are talking about past except 2nd one .All are conditional sentences .
If i had the opportunity... I would visit the whole world . But as I did not get it so I could not . If i have the opportunity... to go to England I must visit Oxford If she/he
If i had the opportunity... I would visit the whole world . But I probably will not get that opportunity now or in the future. If i have the opportunity... to will go to England. I must visit Oxford. If she/he were ( colloquial was) present here she would manage the situation now. If i
I've recently seen a movie, The Hunger Games, and it is/was great. Would using 'is' imply that the movie is still great?- Yes, and 'was' relates to its quality when you saw it.
So, by using 'was', it means that i was great then but not anymore?-- No, of course not. Past tense speaks only of the past; it tell us nothing about the present.
What about this? Since when were/are you a police officer?-- This does not deal with 'if' clauses. 'Were' refers to a past occupation and 'are' refers to a present occupation.