讀到一篇新英的某段落有個看不懂地方，以下為節錄
Two years ago North Korean agreed to close its main nuclear ......
and reveal its nuclear assets, but has since refused to negotiate.
問題是 but has since refused to negotiate. 請問這句的原句長什麼樣子
被中間那個since 搞糊塗了
請文法高手幫我解釋一下了 謝謝!

http://news.bbc.co.uk/2/hi/8066615.stm 看原文就知了。