I think doing more exercises is a good sport for students.请问,谁能告诉我be动词为什么用is,急

宾语从句（Doing more exercises 主语）（is a good sport for students系表结构）.
单个非谓语动词（限于动名词和动词不定式）作主语时,谓语动词用单数.这是语法规则.再如
To look after these students is my duty this afternoon.（照看好这些学生是我今天下午的工作.）