ok thank you !! I'm asking that because when I did an english exam I had to pick between "so difficult" and "such a difficult", but to me they had the same meaning...

This was not such a difficult question. This was not so difficult a question that some might have missed the obvious use. . The second use can mean "as difficult as to" = "so" . It was just so difficult an exam = emphasis on "difficult" In this case interchangeable.

thank you for you're explanation 😀😀

- "such a difficult exam": "it was such a difficult exam" means "it was a very difficult exam" - "so difficult exam" is not correct, but you can say "the exam was so difficult" meaning "the exam was very difficult", or you can ask "was the exam really so difficult?", which means "was the exam as difficult as you thought it would be?"