私が　にほんごを　勉強したいと　おもったのは、そのしゅみが　りゆうかも　しれない。 わたしが　にほんごを　べんきょうする　きっかけは　そのしゅみかもしれない。

The sentense　私の趣味はXXXXです。is perfectly correct. Then, for the secound one.... I guess you want to say "Learning japanese language is only as my hobby", right? If so, you can say: 日本語は、（ただ）私の趣味のとして勉強しています。 Nihongo wa (tada)watashi no shumi toshite benkyou shiteimasu (I learn Japanese as my hobby) If you want to say, "my hobby is only learning japanes"： 私の趣味は日本語を勉強することだけです。 Watashino shumi wa nihongo wo benkyousurukoto dakedesu Knowing lots of vocabularies is a good start, but I suggest you to learn how to construct proper japanese sentenses before you go to the next level. 　 *************************************************************************************************** Hi Yuki, I have recieved your comment,so I am going to give you some sentenses in japanese for the response: "That hobby may be the reason why I wanted to study Janpanese." in Japanese; 多分その趣味がために日本語を勉強したかったのかもしれません。 (Tabun sono shumi ga tameni nihongo wo benkyou shitakatta nokamo shiremasen) You could say 多分その趣味”の”ために　as a preposition. Best of luck! がんばれ！

もしかしたらその趣味は日本語を勉強しようとした理由かしら。 I understand exactly what you're trying to say. I'm not sure though whether the use of かもしれません is correct here. I hope some native Japanese speaker can express this mood a lot better. It's a tough one.