This isn't quite the full story, though. If you're editing Combinatorics.java, but your partner has committed changes to the same file while you've been working, you can't commit. You don't want to unconditionally delete their changes without looking at them first, so SVN won't let you simply overwrite their changes with your version.
To fix this, svn update before you commit. This ensures that you're working on the most recent revision. Even if you and your partner have both made changes to the same file, SVN just merges your partner's changes into your file without you having to do anything about it.
Well, usually. And that brings us to....