<p>when is it appropriate to use has/have/had and when is it just unecessary?</p>
<p>Has and have are of the same tense (present perfect). That tense applies to a situation that has occurred in the past and still occurs now. </p>
<p>For instance, "I have stood here for two hours." I was standing here two hours ago and I continue to stand. "Ever since I graduated from Harvard I have gotten great job opportunities." I used to get good job opportunities, and I still do. This tense can also sometimes take the place of the past tense, though there is a slight difference. For example, "I did my homework" versus "I have done my homework." (Although both convey the same information, the latter conveys a past event in the present tense, and the former conveys a past event in the past tense.) The SAT won't test you on the present perfect in this kind of situation, though.</p>
<p>Had is the pluperfect tense (also called the past perfect). This tense applies to an event in the past that occurred before another event in the past did. </p>
<p>For example, "By the time I left for school, I had already wasted ten minutes watching the news." Both the leaving and wasting are events in the past, but the wasting happened first. "By the time" is also a good clue that the pluperfect will be needed. "I had just finished my memoir when my computer lost power." The finishing came before the losing, thus the pluperfect is used.</p>
<p>Generally, these come up as errors on the sentence correction questions and answers on the identifying errors questions.</p>