I want to write to a professor and tell him about my research interests. What is the best way to say that my research interests include, but are not limited to X? Is the following sentence a good choice? "My main research area is X, but I am open to other subjects as well. "
I guess the answer depends on what kind of information you want to convey, and for what reason you want to make it clear that your interests are not limited to X area. "My research interests include X" is already non-exclusive - just because you say that you are interested in X doesn't mean that you aren't also interested in Y and Z, and most professors realize that students (and all their colleagues) have more than one small narrow area of interest. So you can just say that and be fine.
Basically, my point is that it's absolutely fine to tell a professor that you are interested in one specific area, as long as that area is sufficiently broad in your field. The goal is to find a professor whose interests match or are complementary to yours, so when you say "I am interested in X" he thinks "Ah, I have always wanted a student who would study X!" or at least "Hmmm, it might be good to have a student in my lab/group who studies X."
However, I would not say "My main research area is X, but I am open to other subjects as well." That makes it sound like you would be willing to change your area of interest if the right PI came along or to get into a specific program, and you don't want to convey that, as counterintuitive as that may seem. It makes you sound wishy-washy. You want to be regarded as a serious scholar with a deep interest in a particular area.
Well I think telling profs that you are only interested in a very narrow research field may seem like you are not flexible, and in conduction research you sometimes need to be flexible since your specific research area might be eliminated or replaced according some new findings! But telling them I'm interested in X, but I am open to Y as well seems like all you want is the D*mn fund and admission. ;) (And yes we're are looking for that, but there are better ways to ask for it). You can make it seem more exciting and show yourself as an adventurist to them, not someone who has not made his mind yet. So if you send them the herald that I have made my mind but as I am an adventurist and researcher I am ready for dramatic shifts if necessary you could use some statements such as this one :
I am very interested in X, but science has been always full of surprises for me and I will welcome new exploring options during my graduate studies as I have always done about previous opportunities.
just check out the syntax and grammar ;) since English is not my native language.
I might go with:
Right now, I am very interested in X and Y, but since I'm still a [undergraduate / masters] student, I'm sure that might change, and I want to be flexible and keep my options open for now.
if you are already working professionally than replace * student with early in my career.
This conveys flexibility while admitting adaptability.