i am glad to be at your service.句型上（主谓宾）怎么分析?还有为什么是用to be at

I am glad to。。。glad to 什么呢?I am at your service。I am glad to (that I) [am]be at your service。at your service 是一个词组,跟to be无关,不能看成是不是to be at!glad to是一个动宾式be是为衔接主句后从句的谓语。