"Tough" is a synonym for "difficult" in most cases. Example: That test was difficult! = That test was tough! Example: It's been a difficult year. = It's been a tough year. But, "difficult" is not necessarily always an appropriate synonym for "tough". So, you should be aware of the other definitions of "tough:" it is used to describe things that are resilient and also to describe people who are rather unsavory characters! Example: The meat was tough. -- in order to use "difficult" in the sentence, you would need to clarify it: The meat was difficult to cut. Example: He is a tough guy. -- this basically means someone is not afraid to get into a fight! But most of the time you'll find that they are interchangeable!