sgml parsing with sax parser

In my homework I should parse some sgml files. I using SAXParser. it works correctly for a simple xml file. but when I want to parse homework sgml files this error occur:

Exception in thread "main" org.xml.sax.SAXParseException; systemId: file:///C:/Users/MarkaZ%20Computer%20RooZ/Documents/workspace/HW_02_IR/lewis.dtd; lineNumber: 2; columnNumber: 17; A '(' character or an element type is required in the declaration of element type "LEWIS".

I dont have any education from dtd documents. my code is:

 SAXParserFactory parserFactor = SAXParserFactory.newInstance();
            SAXParser parser = parserFactor.newSAXParser();
            SAXHandler handler = new SAXHandler();


parser.parse(new FileInputStream("reut2-000.sgm"), handler);

How can I prevent this error?

excuse me for bad English