Although you include a correct answer, in everyday English the most common opposite of "easy" is "hard." When President John F. Kennedy gave a speech announcing the United States' plans to send astronauts to the moon, he said: "We choose to go to the Moon in this decade and do the other things, not because they are easy, but because they are hard." The US Foreign Service Institute says that for English speakers, Tagalog is considered to be one of the "hard languages" to learn.