So sometimes, using present perfect is the only correct way to say something. But very often using present perfect is just one of the ways to say something. The problem is that sometimes students are incorrectly taught that only present perfect is correct in a certain sentence, when in fact simple past tense is also correct in that sentence.
Last edited by 2006; 10-Jun-2010 at 02:15. Reason: addition/clarification
You don't need perfect tense in sentences like 'I sent the letter this week.' Adding "have" adds nothing to the meaning of the sentence.
If simple past tense expresses your meaning clearly, that's all you need.