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> What is my hobby?

You can start with saying something about what a hobby is. 

.....Um,  I think my hobby is studying. 

My hobby is studying. 

Um is for informal sentences and used for speaking. Do not use it in writing essays. 

I am not a very intelligent person, but I want to understand the principle of the world and the law of nature.

I am not a clever person, but I want to understand the principle of the world and the law of nature.

Replace very intelligent with another word, or a higher level word. 

I also desire to become a wise person. But I do not practice my hobby constantly, because my mood is unstable. 

My another hobby is playing the piano. I like classical music, because it gives me deep and rich inspiration. I can't play the piano often,either. 

I 'm not so good at playing the piano because I don't have enough time to do it. But I hope to play the piano very well some day.

A sentence should not begin with but. 

some day - someday

the piano - piano

be mindful of the spacing

often, either - often, either

My third favorite hobby is listening to the radio. It connects the world and me. I usually listen to the radio while washing the dishes or cooking food or sewing the clothes. The good point of the radio is  that I can do other things at the same time. Nowadays I enjoy listening to MBC FM channel. The programs of this channel is easy, interesting, and full of very useful information. 

My third favorite hobby is listening to the radio. It connects the world and me. I usually listen to the radio while washing the dishes, cooking food, or sewing the clothes. The good point of the radio is  that I can do other things at the same time. Nowadays, I enjoy listening to MBC FM channel. The programs of this channel are easy, interesting, and full of advantageous information. 

be mindful of the use of punctuations

the subject should align with the verb