<a href="http://grammar.ccc.commnet.edu/grammar/sequence.htm"><a href='http://grammar.ccc.commnet.edu/grammar/sequence.htm' target='_blank' rel='nofollow'>http://grammar.ccc.commnet.edu/grammar/sequence.htm</a> </a><br/><br/>This credible site says to use the past tense for any purpose when the main clause is either the present or past perfect.<br/><br/>There seems to be execptions, more than a couple too:<br/><br/>Eg.<br/><br/><i>Things <b>have changed</b> while you <b>have been</b> away."</i><br/><br/>Is this site wrong or what?<br/><br/><br/><br/><br/><br/>Thanks