Diode transfer characteristic question?

We have the limiting circuit with diodes.The diodes have Rf=20 Ohm, Rr=infinity.Vγ=0.
a) Build the transfering characteristic V0=f(Vi)
b) Make a full explanation for every characteristic zone ( The state of the diodes,the values of the tensions in the entrance and in the exit,etc)
c) Replace the diode D2 with the resistance R.Build the transfering characteristic in this case.Compare with the previous result.
d) Change the Rf parameter of the diode to create only one breaking point.(So to pass from the characteristic of the point A to the characteristic of point C)
http://i.imgur.com/cixtsbM.jpg

a)I dont know how to build the transfering characteristic.
b) The D1 is forward biased,therefore there is a short circuit and zero voltage to it.D2 is reverse biased therefore i=0 and the voltage in it is 20 V.The value of Vi is 0 and V0 is 20 V. (I dont know what other zone of the circuit should I explain here.
c) d) I dont know how to do these either..
can you help me?