Most Subversion commands take the --username option to specify the username you want to use to the repository. Subversion remembers the last repository username and password used in each working copy, which means, among other things, that if you use svn checkout --username myuser you never need to specify the username again.
As Kamil Kisiel says, when Subversion is accessing the repository directly off the file system (that is, the repository URL is of form file:///path/to/repo or file://file-server/path/to/repo), it uses your file system permissions to access the repository. And when you connect via SSH tunneling (svn+ssh://server/path/to/repo), SVN uses your FS permissions on the server, as determined by your SSH login. In those cases, svn checkout --username may not work for your repository.