If you have a chance to go to Australia or USA and stay there for a few years, I believe your English must be very good, however, I am afraid that you don't have this chance.
"If you have the chance to live in Australia or USA for one to two years, I believe that your English will improve. But I fear that you do not have such chance."
If you have a chance to stay at Australia or USAa few years, I believe your English must be very good,but, I am afraid that you have no chance.
If you have (条件) a chance to go to Australia or America and stay there for one or two years, I believe your Englisn would certainly be quite good. But I am afraid you would not have this kind of chances.
If you had(虚拟语气) a chance to go to Australia or America and stay there for 1 to 2 years,i believe that your English would become excellent。however,i am afraid that you don't have this chance.
If you have a chance to Australia or America and live there for one or two years, I believe your English will be very good.