Most of the time, "fashion" is a noun (a subject or object) and "fashionable" is an adjective (describes something). Examples: She's wearing a very fashionable dress tonight. (adjective, describes the dress) I don't understand fashion. (and object of understanding)

fashion = noun fashionable = adjective It's in fashion. It's the fashion. It's fashionable. These three all mean the same.

Someone,who is 'fashionable' (the adjective) ,is wearing the latest 'fashion' (the noun)!

piensa en el fin de la palabra "fashionable" -- "able" esto como una palabra en si me dice "capable" entonces "fashionable" para mi quiere decir que la cosa a que se refiere es "capable de ser fashion"