I'm polish, in school we always called it Poland. I know that it was made up of the merge of Poland and the Grand Duchy of Lithuania but it was ruled by one king, under one flag, and had basically one capital. It was one country for several hundred years so why is in not simply refereed to as Poland?
It might probably a stupid question but never the less I want to know why is it universality refereed to as the commonwealth, outside of Poland.
I know that it was made up of the merge of Poland and the Grand Duchy of Lithuania but it was ruled by one king, under one flag, and had basically one capital
Only it wasn't. "One king" also used the title of Lithuanian Grand Duke. Both Lithuania and Poland had separate (and different) administration systems and governments. The commonwealth, for instance, had at a given time two treasurers (podskarbi), two chancellors and four hetmans.
Only the constitution of May 3rd abolished the division between Lithuania and Poland.