请问老师：I dont want to get too absorbed in modeling.It is certainly fun but the lifestyle is a little u

certainly在这里是作状语，不是作定语。意为无疑、一定。所以用副词。比如，He is certainly a good student. 

形容词certain 作定语，意为某个。比如，A certain gentleman is waiting for you outside. 

虽然fun作表语时为名词，但certainly不定式修饰fun的，而是修饰系表结构的。