Basically, you just need to relax your ideas a bit: words are divided into general word classes: verbs nouns adjectives adverbs prepositions determiners-- have I forgotten something?-- but these are not rigid hermetically sealed confinements, except maybe for basic prepositons. Words flip classes all the time.
'To study' is here a verbal noun: it has characteristics of a verb, and a noun. As a verb it takes an object, 'English', As a noun it forms a compound noun with another noun 'English'. This compound noun is then described with the adjective 'difficult'.