I hate waiting long hours in the hospital，only to spendfive minutes with an indifferent doctor.
1 这里的only to是什么词性的词组，是副词还是连词？
2为何要在only前面加逗号呢？
副词 only 后接不定式，表示结果（结果状语），指不符合句子主语意愿的结果（特别指出人预料的结果），这种使用方法前面一般有逗号。又如：
He worked very hard, only to failagain. 他努力地用功了，结果还是失败了。
I hurried to school, only to find outthat it was a holiday. 我急匆匆地赶到学校，结果却发现那天放日。
He spent four years studying, only tolearn that there were no jobs. 他花了四年时间学习，却发现找不到工作。
He returned, only to discover his carhad been clamped. 他回来了，不料发现我们的汽车被上了夹钳。
有时如此用的不定式也表示目的。如：
I didn’t mean to hurt her, only tokeep her still. 我没想要伤害她，只不过想让她别动。（柯林斯词典）