I would like to know the differences between the words "difficult" and "tough".
How differently to use both of them?
Here are two some sentences extracted from a book.

The difficulty about portrait-painting is that you can't start painting portraits until people come along and ask you to, and they won't come and ask you until you've painted a lot first. This makes it kind of difficult, not to say tough, for the ambitious young man.
tough: very/extremely difficult

Please note also that the term '(very) difficult' would be more commonly used than 'tough' in formal writing/speaking.

Best wishes, Clive