I take the trouble to return it.
这里的不定式 to return 是在句中当副词做状语修饰take么?就譬如 I want you to behappy.
take the trouble to do 不辞辛劳地做 ... He kindly took the trouble to see me home. 他不辞辛苦地把我送到家。(《英汉多功能词典》新编升级版p.1829)
可以把这里的 to do 看作目的状语。
I want you to be happy. 这里的 to be happy 是宾语补足语。