若but后有主语（无论整个句子有一个或多个主语，主语是否一致），在but之前加逗号。

例：McCain ran for president, but I did not vote for him

　　若but后没有主语，不要在but之前加逗号。

　　例：McCain ran for president but did not win.

　　文中两句子可能属于并列关系，故用逗号。否则应该换成however或去掉逗号.