If I make an own folder for the subdomain and I install b2evo there, multidomain feature doesn't work.

You can call a blog bu name so [example.com/blog1] and [example.com/blog2] are ok
Then if you can make subdomains via surely you can set up a redirect so that [blog1.example.com] will go to [blog1.example.com/blog1]

If your base URL is http://subdomain.example.com/, then using the following for collections:

• http://subdomain.example.com/ is super easy
• http://example.com/collectionname/ is also super easy
• a combination of both as in http://subdomain.example.com/collectionname/ should also work but may require more clicking to configure.

In any case: there must be a single b2evo install and a single index.php file. ALL domains and subdomains must point to the save root folder and same index.php on disk.

If you get a 404 page you must look at it in details to see if it was generated by b2evolution or by your webhost.

" combination of both as in http://subdomain.example.com/collectionname/ should also work but may require more clicking to configure."

I would like to solve this. could you help me?

I am afraid this is too specific and needs too much testing specific to your private hosting. I don't have that much free time to investigate your particular case.

Nonetheless you will find the relevant documentation in this section: http://b2evolution.net/man/back-office-reference/collections/collection-settings/urls-settings/

no matter what, I strongly suggest you start by configuring your baseurl to http://yourdomain.com/ (without www). Then try to use 'Absolute URL:' for each of your collections.