My knowledge is limited on the subject but I will try and look forward to being corrected by some knowledgeable members.
### command to implement RewriteCondition AND RewriteRule
RewriteEngine on
### reads command line and interprets as a file
RewriteCond %{REQUEST_FILENAME} !-f
### reads command line and interprets as a directory
RewriteCond %{REQUEST_FILENAME} !-d
### interpret, execute and route RewriteCond to specific root file including directories
RewriteRule ^(.*)$ index.php/$1 [L]
### interpret, execute and route RewriteCond to specific root file excluding directories
RewriteRule ^(.+)$ index.php?sponsorUsername=$1 [L]