我想你想问的是
主谓一致这一语法现象中的
就近原则。
当
there be 句型的主语是一系列事物时，谓语应与最邻近的主语保持一致.
There is a pen,a knife and several books on the desk..
There are twenty boy-students and twenty-three girl-students in the class.
总的来说，在由not only…but also…，not just…but…，or, either…or…，neither…nor…连接主语的句子中及在there be句型中，
谓语动词的单复数按就近原则处理，即按与谓语动词最靠近的那个主语来确定谓语动词的单复数形式。如：
Not only John but also I am going to Shanghai next week.
Either you or she is to go.
There is a pen,a few envelops and some paper for you.