Hi Jaaguar, I think that's an interesting question. See, Japanese is a funny language. Intonation is very important for this tricky language. As you might know, Japanese relies a lot on formality indicators (for example, less formal or more formal sentence structures). Once these two clash (formal and informal expressions on the same sentence), it creates a nuance that may be seen as offensive or cynical, as if one is mocking another. In this sense, you are right that "I study Japanese." is "Nihongo o benkyōshiteimasu." But, if you say "Nihongo o benkyōshiteiru dake desu", it may be seen as offensive or cynical. As I mentioned before, because "dake" is often used in informal sentences (with some exceptions), if you use "desu" after that, which is a little more formal, it creates the clash that I was talking about. However, you can easily avoid the misunderstanding by just adding "yo" at the end of the sentence. (Nihongo o benkyōshiteiru dake desuyo) (I'm sorry if this is not what you were asking about, but I believe this might help you in the future). So moving on... A QUICK ANSWER TO YOUR QUESTION: 1) "I am only studying Japanese." is "Nihongo dake benkyōshiteimasu." (If you mean "no other languages, only japanese) or 2)"I am only studying Japanese" is Nihongo o benkyōshiteiru dake desuyo" (for formal way) or "Nihongo o benkyōshiteiru dake dayo" (for informal way).