Student or Learner
I would like to know the difference in the meaning between "1" and "2".
I'm upset because my cat is sick.
I'm sad because my cat is sick.
"Upset" covers a wider range of emotions, including being sad. I may be upset thinking of the big vet bill that is waiting for me. I may be worried that he won't recover.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.