I have a domain and two subdomains.They refer to the same host.(http://www.171english.cn,http://myenglish.yi.org, http://elt.008.net) I'm not sure if that will hurt my future pr of http://www.171english.cn.Will that be punished by search engines?
subdomains and domain names are threated as two separate entities when google decides on which pagerank to give. in fact blogspot.com has a very high pagerank but all subdomains with this site have a totally different pagerank. If you mean that does the domain affect the subdomain? than the answer is no
Subdomains will not affect your PR. Go and read more about "What reduces PR" search for it on Google. All the best.