How many times is a word being searched? Does any one know how to check it out? Is there a way to find out how many times a particular word or phrase is being searched for on google? Any links? Do you know how backlinks from domain affects subdomains and is it better to focus on backlinking domain or particular subdomain?
1. Just as Tiggerito mentioned above, Google adwords keyword tool would be right choice for your purpose. Another thing I would like to mention would be factor KEI(keyword effective index) would be another essential issue for targeting right keywords.
2. If you have links on your subdomains back to your main domain, then building back links for your subdomains would be really helpful for your main domain ranking.