What is the difference between 突然 and 忽然? They both have the meaning sudden, but what is a subtle difference between 突然 and 忽然?
They are identical when used as an Adverb.  


  天空突然/忽然下起了大雨。


But 突然 can also be used as a complement of a verb. 


  这雨下得太突然了。 


Where you can only use 突然。