[quote=Lenka;136683]I know what the reason of not using present perfect in the sentences is already.
The (shall I use the definite article here, by the way?)
either would work present perfect tense can be used only with the following prepositions:when, after, until, as soon as. => but not "before"!!
I'll come as soon as I've finished it. => You have finished it first and after that you'll come. (correct me if I've made any mistakes in my English, please!)
Quote:
Why not 'before', Lenka?
Okay, I'll come before I've finished it.
It's not the norm, of course, but in certain circumstances, it's possible.
Let's go home before it starts (not "it has started") raining. => We'll go home and after that it will probably start raining. It isn't supposed that it has started raining and after that we'll go home...
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Consider a situation where someone always waits too long and gets caught in the rain. Another person, being a wee bit or a lot strident, commenting on such a situation, could well use the present perfect;
This time, why don't we go home [OR] let's go home BEFOOORE it has started raining.