XML tags are

case-sensitive

- because of the case difference in two tags, which is treated as incorrect syntax in XML

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<student>
   <name>George</name>
   <city>Tbilisi</city>
   <phone>(011) 123-4567</phone>
   <address>Wrong syntax</ADdress>
   <hr />
</student>

This will generate the error:

error on line 6 at column 23: Opening and ending tag mismatch: address line 0 and ADdress

XML tags must be closed

in an appropriate order

, which means an XML element opened inside another element must be closed before the outer element

<outer_element>
   <internal_element>
      Right, because the inner tag is closed before the outer
   </internal_element>
</outer_element>