初学javascript,请教怎样用javascript控制一段HTML是否执行

<html>
<SCRIPT LANGUAGE="JavaScript">
if(a) 
{</SCRIPT> <a href="../Visit/QueryMenu.aspx" target="_top" onClick="MM_nbGroup('down','group1','menu_out_v','../Common/Img/menu_over_v.jpg',1)" onMouseOver="MM_nbGroup('over','menu_out_v','../Common/Img/menu_over_v.jpg','',1)" onMouseOut="MM_nbGroup('out')"><img src="../Common/Img/menu_over_v.jpg" alt="" name="menu_out_v" width="135" height="30" border="0" onload="MM_nbGroup('init','group1','menu_out_v','../Common/Img/menu_out_v.jpg',1)"></a>
 <SCRIPT LANGUAGE="JavaScript">
}
else
{</SCRIPT>
<img src="../Common/Img/menu_over_v.jpg" alt="" name="menu_out_v" width="135" height="30" border="0" onload="MM_nbGroup('init','group1','menu_out_v','../Common/Img/menu_out_v.jpg',1)"><SCRIPT LANGUAGE="JavaScript">
}</SCRIPT></html>