Completed - is 'simple past', it just means he did it. Has completed - you are looking at an event in the past from from today's point of view. This person, as things stand today, has indeed completed it. He completed it in the past, but we are looking at it today, whether he has done it or not. So for example a store may be giving gifts today to anyone who has completed the survey. Had completed - you are looking at it from yesterday's point of view, or some time in the past. At that time, when the store was giving out gifts, they looked at whether the customer had completed the survey or not. If they had done it, they got the gift. So we are looking at it from the point of view of that person in the past, not from our point of view today. So that is why it's past (had), because we are looking at it from the point of view of people at that point in time in the past. Here are some helpful links: http://www.englishclub.com/grammar/verb-tenses.htm http://www.englishclub.com/grammar/verb-tenses_sys-tenses-regular-verbs.htm