tough vs difficult

tough is a noun but difficult is not a noun.

Word	Noun	Adjective	Verb	Adverb
tough	Yes	Yes	No	No
difficult	No	Yes	No	No

tough (noun)	difficult (noun)
an aggressive and violent young criminal
a cruel and brutal fellow
someone who learned to fight in the streets rather than being formally trained in the sport of boxing

tough (adjective)	difficult (adjective)
resistant to cutting or chewing	not easy; requiring great physical or mental effort to accomplish or comprehend or endure
physically toughened	hard to control
not given to gentleness or sentimentality
unfortunate or hard to bear
substantially made or constructed
feeling physical discomfort or pain (`tough' is occasionally used colloquially for `bad')
violent and lawless
very difficult; severely testing stamina or resolution
making great mental demands; hard to comprehend or solve or believe