Simple proposal for the exercise in this thread: http://www.python-forum.de/viewtopic.php?f=1&t=30699

tax.py

#!/usr/bin/env python

# was auch immer das sein mag...
CRITICAL_LIMIT = 4000


def calc_tax_value(salary, tax_rate):
    return salary * tax_rate / 100


def calc_tax(salary, married, limit):
    if married:
        if salary < limit:
            return calc_tax_value(salary, 22)
        else:
            # Was auch immer hier hin muss
            return 0
    else:
        if salary < limit:
            return calc_tax_value(salary, 26)
        else:
            return 0


def main():
    salary = float(input("Gehalt in Brutto:"))
    married = input("Familienstand (verheiratet / ledig):").lower().startswith("v")
    print("Der monatlich zu zahlende Steuerbetrag betraegt {} Euro.".format(
          calc_tax(salary, married, CRITICAL_LIMIT)))

if __name__ == "__main__":
    main()