Try this:
# Pretty URLs -- Uncomment to use
RewriteEngine On
RewriteBase /sp/subjects/
RewriteCond %{QUERY_STRING} letter=By%20Department [NC]
RewriteRule
(.*
) /subjects/staff.php?letter=Departments [R=301,L]
RewriteCond %{QUERY_STRING} letter=Subject%20Librarians%20A-Z [NC]
RewriteRule
(.*
) /subjects/staff.php?letter=Subject\ Librarians [R=302,L]
RewriteRule ^staff/
([^/*
]+
)/?$ staff_details.php?name=$1 [L]
RewriteRule ^profile/?
([a
-zA
-Z0
-9\[\]\.()+|_-
]+
)$ staff_details.php?name=$1 [L]
RewriteRule ^
([^/\.
]+
)/?$ guide.php?subject=$1 [L]
RewriteRule ^
([^/\.
]+
)/
([^/\.
]+
)/?$ guide.php?subject=$1&source=$2 [L]
RewriteRule ^
([^/\.
]+
)/?$ guide.php?subject=$1 [L]
ErrorDocument 404 /sp/subjects/blank-404.php