Once you create a subdomain, should I submit it to Google? If not, what if there is no link to the subdomain from your main domain? Thanks for any help!
No need to submit, google can indexed your site even you don't submit it. Let them find your site by linking your site from other site that is frequently updated with new content or you can submit your site on bookmarking site.
I think it'll be better if you put your subdomain link to your main domain. because your main has been crawled and indexed already. And just like what the posters mentioned above^ building links with your subdomain will help your site get indexed faster especially if it is related. no need to resubmit again.
It is better to have good link structure for your site, build links for subdomains... and a good sitemap could help too.
I think you can submit but there should be some external links to your subdomain to help Google index faster..
If google finds sub domains naturally then it's best. If in 2-3 months google boats don't find your sub domains then submitting for crawl is ok.
If there's no link to the sub-domain it wouldn't matter as long as in the source code it says "index". If the domain is pretty well ranked or has incoming links, then the sub-domains should automatically get listed.