Sure. Keywords in domain names can contribute up to 5% to your search engine ranking. If you have an existing website and want to change the domain name on that existing site, make sure that you have 301 redirects set-up to point all previous pages to their corresponding URLs on your new domain.
No don't use in your main domain, that will affect by google Algorithm EMD (Exact Match domain), but you can use in the sub domain page of your main domain. Eg. http:www.domainname.com/keywords.php
Yes, keyword/s are very important to help your target users, customers to visit or easily identify your website or campaign.
If you can, but it depends on your site. If you are going to do enough work with branding and advertising then you get around any problems there might be with the domain name and keywords.
use Keyword on Domain is better to give relevancy about on-page content and crawl by search engine to add niche power
Ideally, webmasters should strike a balance between finding a catchy, unique, brand-friendly domain name and having a domain that contains keywords they are trying to target. The benefit of a keyword-rich domain is two-fold. First, the domain name itself is a ranking factor that the engines consider when calculating ranking order. Second, having relevant keywords in a domain name is beneficial because the domain name is the text that other Internet users will use as anchor text when linking. Since keywords in anchor text are an important ranking factor, having these keywords in a domain name can have a positive impact on ranking.
Based on my research, partial match domains works best. Don't have an exactly matching domain name. Instead, have your domain name as something like www. Keyword+Brand .com NOTE: If the domain becomes too long, then I wouldn't recommend using it.