biorac po rozwage tylko jedno, tzn roznice pomiedzy 'since' a 'for' przy uzyciu czasow Perfect, wyglada to tak:
since - od punktu/momentu/wydarzenia w czasie (az do chwili obecnej)
for - od jakiegos okresu czasu (az do chwili obecnej)
I have written three novels since 1990.
I have been writing novels for 17 years.
I have appeared in a few movies since I turned 18.
I have been appearing in movies for a long time.