How can I show my enthusiasm for my subject?

The best way to do this is prove your interest. Rather than simply saying that you are passionate, demonstrate it in a way which shows you have gone beyond what you would be expected to do in school. You could for example read some extra books on a topic of particular interest to you. When you write about them, be specific and refer to the authors and ideas from the text so that your application really gives a sense that you've read the material. To make it even better, show that you can engage critically with the topic by saying if you agree/disagree with the writing and why. This will show that not only are you motivated enough to go the extra mile but that you think about what you've read, its strengths and weaknesses. The key to this is showing that you care enough to go above and beyond other applicants but also showing that your words are supported by actions and not something that you've said to try and make your personal statement seem more impressive.