I have a Japanese assignment that requires me to write about anything I want. I decided that I wanted to write about why I want to learn Japanese. I think the title should be "Why I want to study Japanese"; would 私は日本語を勉強したい理由 work? I am not sure if I am using 理由 correctly here.
This is a nice try, but the 「は」 needs to be replaced by a 「が」. 「は」 is not an option here. Why not?
That is because 「私が日本語を勉強したい」 is a relative clause that modifies 「理由」, correct?
Inside if-clauses and relative clauses, the subject/topic marker is always 「が」.
The 「が」 in either sentence cannot be replaced by a 「は」. I might even add that this particular mistake in the particle choice sounds more serious to us native speakers than you might think.
Finally, you are using 「理由」 correctly.