mod_rewrite causes 500 server error/Fails/works partially - .htaccess

Question

I am trying to do a permanent redirect from my old design (www.your-translations.com) to my new design at www.your-translations.com/_YT.
In the process, the urls have to change from this format:
www.your-translations.com/my_page.php
to this format:
www.your-translations.com/_YT/index.php?pge=my_page
I have made many attempts so far, the latest of which is:
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^.*your\-translations.com.*$ [NC]
RewriteRule ^(.*?)/([^/]*)\.php $1/_YT/index.php?pge=$2 [R=302]
RewriteCond %{HTTP_HOST} ^your-translations.com [NC]
RewriteRule ^(.*)$ http://www.your-translations.com/$1 [R=302,L]
This is one of my better tries in that it apparently does almost nothing (previous tries have resulted in 505 errors and infinite loops). The rewriting of URLs to "www." works properly.
I have tested the regex in different regex testers and it seems to do what I expect it to do.
Can someone explain what I am doing wrong?
WHAT exactly is the pattern applied to? Is it what is matched by the RewriteCond? The URI as I typed it in? The relative path? The actual path on the server?
Is there any way to display the string before I try to match it?
Several suggestions found in tutorials result in infinite loops on my server. Do all versions work the same way?
Unfortunately, the webhost technical support doesn't seem to know anything about mod_rewrite, so I can't expect any help from them.
According to my FTP log, I have already made about 80 attempts and I could really use a hand here.

Answer 1

Options +FollowSymlinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^your-translations\.com$
RewriteCond %{REQUEST_URI} !^/_YT/([a-z0-9-_]+)\.php$
RewriteCond %{REQUEST_URI} ^/([a-z0-9-_]+)\.php$
RewriteRule ^(.*) /_YT/index.php?pge=%1 [QSA]
RewriteCond %{HTTP_HOST} ^www\.your-translations\.com$
RewriteCond %{REQUEST_URI} !^/_YT/([a-z0-9-_]+)\.php$
RewriteCond %{REQUEST_URI} ^/([a-z0-9-_]+)\.php$
RewriteRule ^(.*) /_YT/index.php?pge=%1 [QSA]
In RewriteCond, you must type a backslash before a dot and not before dash, because a dot in .htaccess means any character, so you need bslash to call is as dot. Sorry, I'm in mobile, so I couldn't explain more, but feel free to ask what you want to know about the code.

Answer 2

Thanks to some explanations from webmasterworld, I was able to fix my mod_rewrite code:
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^your-translations.com [NC,OR]
RewriteCond %{HTTP_HOST} ^www.your-translations.com [NC]
RewriteCond %{REQUEST_URI} !="^.*?/.*"
RewriteRule ^([^/]*)\.php$ http://www.your-translations.com/_YT/index.php?pge=$1 [R=302,L]
RewriteRule ^$ http://www.your-translations.com/_YT/ [R=302,L]
This seems to work correctly so far (touching wood). Thanks Servant for your help.