Help me to see the difference and understand the reason why a person would choose one over the other.<br/><br/><i>1. Who have you served today?</i><br/><br/><i>(I think this tense is used because the content has a relevance as he speaks.)</i><br/><br/><i>Who did you serve today?</i><br/><br/>Why is it not "three thousands"?<br/><br/><i>About <u>three thousand</u> were added to their number that day.</i><br/><br/>Why would one capitalize the underlined words?<br/><br/><i>Use of this <u>Community Center</u> and its amenities are at your own risk.</i>