arduous
This word is more formal than "hard" or "difficult." You will often see it in writing, such as news reports, history books, or professional documents, rather than in casual conversation. It specifically describes a task that takes a long time and requires a lot of physical or mental energy. While "difficult" can describe a puzzle or a math problem, "arduous" usually implies a grueling process or a long journey.
💬Casual Conversation
🎬Tuesday night, Chloe is procrastinating in her dorm room while Fatima is already at the library.
Chloe Smith
I can't deal. This lit review is way too arduous for a Tuesday.
Fatima
Stop whining and just grind it out. See you at 8.
💡
Chloe uses 'arduous' to dramatically describe the difficulty of her academic work, while Fatima responds with the phrasal verb 'grind it out', meaning to perform a tedious task through persistence.