The rule of thumb that I know about Perfect is that when you use perfect, it doesn't matter when the action was done but what matters is that it was done - the effect.
So when I'm saying
I've done my homework it doesn't matter WHEN I did my homework just the fact that I actually did it.
However, what happens in that case:
- I've already done it last week
- I already did it last week
In both sentences I mention that I did it
last week. Does it mean that the first sentence isn't correct? Or maybe that I should use the second sentence?