For example, do you think that google and other search engines make a difference when one domain name is like www.siteaboutsoccer.com and other named www.site-about-soccer.com ? Is it better to separate the keywords ?
Makes no difference. Search engines can very easily identify dictionary terms regardless of the language and they do not care if there is a space, dash, or nothing in between them.
Do not over think SEO. Please. SEO is not a set of triggers you pull to get better ranking. It simply does not work like that. Yes- there are things to do and not to do, but for the most part, these have largely been mitigated starting in 2008. Just focus on content and customer experience. Everything else you can come up with would be a distraction. Even the Black Hatters are talking about content and customer experience. You should be too.