Chris explained it well, so I'll give some examples. Keep in mind "I have" is contracted to "I've" and "You have" is contracted to "You've". ”You lied to me. Where were you last night?" He's talking about last night, not the consequences today. "You've lied to me [before]. I can't trust you anymore" He's talking about the present consequences of the past. "Do you want to study with me?" "I've already studied" "When did you study?" "I studied last night" The first question, since I have already studied, I don't want to do it today. The topic is about studying today. The second question no longer has anything to do with present, so I use the past tense (without have).

"I lost my key" is in the past tense. "I have lost my key" is in the present perfect tense. The present perfect tense is used to describe a CURRENT state caused by a PAST event. So "I have lost my key" means that I lost my key in the PAST, and the key is STILL lost (in other words: I remain in the state of having lost my key). By contrast, "I lost my key" simply describes a past event. If I find my key, I can still say "I lost my key". This fact about the past does not change when I find my key. However, I cannot say "I have lost my key", because I am no longer in the state (the state of having lost my key) caused by the past event (the event of losing the key). Does this help? Sorry about the complicated explanation. It's a difficult question. There's a good explanation here, if you're interested: http://en.wikipedia.org/wiki/Present_perfect