Couple of important issues before I migrate into Drupal Currently my website is in .NET and I would like to migrate to Drupal in order to manage the content better.
My site currently supports 22 languages having different domain names for each language. The site has the same theme for all of the domains.
Generally, the site (including all of its domains/languages) is divided into 3 subfolders/sections which are responsible for different content and slight changes in the theme. for example:
The URL http://www.example.fr (which is responsible for the French language) is divided into 3 content sections:

http://www.example.fr/sec1/
http://www.example.fr/sec2/
http://www.example.fr/sec3/


and so is the other domains/languages. As a matter of fact, each section contains a slightly different version of the whole site.
I have installed fresh D7 and also I have installed i18n (and its dependencies),pathauto, panels and context modules but I have yet succeeded to answer the following questions:

How can I create a node and assign it to one of the above sections ?
Once I have managed to assign a node to a section, how can I make it have a URL prefixed by the appropriate section prefix like sec1,sec2 etc. ?
How can I have my theme changed according to the section the user is in?

I have heard that Drupal has a very supportive community so I am really looking forward to someone who will be able to help me in these matters as soon as possible.
So thank you all.
Taxonomies and pathauto/token will solve your first two questions. Use a taxonomy to categorise your nodes, then use a token such as [term] or [termpath] to add it to the start of the automatically generated URL paths.

The sections module will solve your last question. You can assign different themes based on paths, or for individual nodes, and some other ways too. Unfortunately, it looks like it doesn't currently have a released D7 port. There may be one in the works, or you may need to try to port it (likely not hard from my understanding of how it works). Possibly related project is taxonomy_theme, which seems to have a -dev D7 release. I haven't used this module in either D6 or D7 personally though; I know sections works fabulously though in D6 despite not having an update in 2 years now.