Why is in the following sentence only "seit" the correct form? Why is "für" false?
Anna ist seit einer Woche fertig mit der Schule.
Why is für eine Woche not correct here?
As far as I know, seit refers to an action that has already started in the past and is still going on and für refers to actions which started and ended.
In the above example, Anna has finished/finished school a week ago. This action has ended but the book says seit is the correct form. I don't get it!
The graduation is an event, and the action of going to school is finished. Thus it has to be 'seit'.
However 'für eine Woche mit der Schule fertig' is not wrong. It has a completely different meaning though which you probably don't mean: it implies that going to school is only temporarily done for, thus the being done is a continuous event with a foreseeable end: Anna's class is doing a 2-week project time, and she is a quick and digilent worker so that she finished all her tasks within one week and NOW on Monday is done with it for the week: Anna ist für diese Woche mit der Schule fertig.
Generally since and for in a temporal sense are used way different in English than seit and für in temporal sense in German: Both English constructs translate to 'seit', always. And 'für'is only used in German for finite time intervals. Four examples with typical pitfalls:
Anna is working at the new job for two weeks = Anna arbeitet auf der neuen Stelle seit zwei Wochen
Today is May 1st. Anna works here since April 1st,thus for four weeks =Heute ist der erste Mai. Anna arbeitet hier seit dem ersten April, das heißt seit vier Wochen hier.
In June Anna will fly to Japan for two weeks = Im Juni fliegt Anna für zwei Wochen nach Japan
I'm done with the tasks for today (but tomorrow will there will be more) = Für heute bin ich mit den Aufgaben fertig (aber morgen gibt es weitere)
You have to read "fertig sein" as "to have finished" or "to be a graduate" here, not as "to finish". She finished school a week ago, so her "action" of being a graduate started then and still goes on – hence "seit" (and present tense).
These two sentences are correct, but mean different things:
Anna ist seit einer Woche fertig mit der Schule.
Anna finished school on July 1st, now is July 8th so it is a week since Anna finished school.
Anna ist für eine Woche fertig mit der Schule.
Anna finished school on July 1st, now is July 3rd and on July 8th her first course of the university she enrolled into will start. But for the span of a week Anna is finished with her (current) school.
As you can see the words seit (since) and für (for) do not work too differently from their english counterparts.
