I began to work in Beijing in the year when New China was fo

是定语从句.when引导的定语从句和状语从句的区别在于是否有修饰关系,也就是能否在主句中能否找到先行词,若能则是定语从句.when引导的状语从句一般意为：当.的时候,.样.
例如 本题的先行词是the year,翻译是我开始与北京工作是在（新中国成立的）那一年.New China was founded修饰the year,所以本题是定语从句.
I feel very happy when you come to see me. 是状语从句,无先行词,意为【当】你（们）来看我【的时候】,我感到很高兴.