İsim fiil: We turn verbs into nouns with -me, -iş, -mek suffix. It's simply like gerunds, to run or running. (we make verbal nouns. you can think as we give a name to actions.)
Sıfat fiil: We turn verbs into participles ~ adjactives which gives more information about the noun it's connected to. we use -en, -esi, -mez, -ar, -di(k), -di(ği), -ecek, -miş suffix. These words generally makes adjective clause.
This is a subject even our 8th grade students (12-13 year old native speaking children who are preparing for high school exam) feel difficulty. Even they use these words in daily basis, understanding the rule and applying them with purpose is an advanced level of proficiency.
But it's not "difficult" or "impossible to learn". Just needs more exercise and experience since there is not one solid suffix gives one exact meaning.