Hello :-)
I can not extract some rar files and have this messages : The archive is either in unknown format or damaged or The file is not a valid TAR archive..
This files were downloaded from the internet
How to resolve this a problem ? Is it possible to repair the damaged rar files and how ?

You can repair the files with other files called "Par" files. That is if they were provided with the rar files. They can rebuild bad rar's.