真剣 [しんけん] – seriousness; a real sword

• 真 [シン・まこと] – true; reality
• 剣 [ケン・つるぎ] – sword

真剣 is a real sword.  Not a toy.  Not a replica.  Not a wooden training tool.  真剣 is a real, live, sharp blade that can maim and kill.  The notion of such a dangerous instrument gives a clear impression of something very serious.

To change this word from a noun into an adverb, simply add に: 私は日本語を真剣に勉強しました (lit – “I studied Japanese seriously” or “I made a very serious study of Japanese”).  This rules works for changing many nouns into adverbs.

One of my favorite 四字熟語 is 真剣勝負 [しんけんしょうぶ] – “a fight with real swords.”  It indicates that the effort being described is extremely serious, even to the point of life-and-death.