David R. answered 05/11/19
Japanese tutor since 2008.
Hello! I'm having a little trouble understanding your question, but I think I understand.
I think 難しい勉強をしました works better (I did a difficult study), but that might change on a case-to-case basis. The material studied was difficult, so I believe 勉強 becomes a noun.
お父さんはしっかり運転します。Father drives steadily. The action here is "driving," so there is an adverb.
今朝、元気に散歩した。I walked energetically this morning. Walking, the action, is modified.
複雑なフランス料理をしてみました。I tried to make a complex French recipe. The recipe (or way of cooking) was complex, so it becomes a noun.
I hope this helps...