validation - Any help why doesn't the XML validate to the DTD -

dtd:

<?xml version="1.0" encoding="utf-8"?>  <!element language (definition, tu, tuv, seg) > <!element definition > <!element tu > <!element tuv > <!element seg (#pcdata) > <!attlist tu id cdata #required > <!attlist tuv lang cdata #required > 

xml:

<?xml version="1.0" encoding="utf-8"?> <!doctype language system "language.dtd">  <definition> # line seems problem     <tu id="webpage-title-text">         <tuv lang="en">             <seg>demo cms</seg>         </tuv>     </tu> </definition> 

the error message is:

document root element "definition", must match doctype root "language". [4]

my questions:

1. what reason xml not considered valid against current dtd?
2. what have missed in dtd?
3. could recommend place take resource of learning how use dtds, schemas , etc.

change

<!doctype language system "language.dtd"> 

to

<!doctype definition system "language.dtd">