[BASIC] How to interpret this circuit (including diodes), need detail explanation

This thing is clamp circuit from this website: http://www3.nd.edu/~lemmon/courses/ee224/web-manual/web-manual/lab5/node9.html. I dont understand how it works or maybe in half way. Could someone provide detailed solution (in horse sense )? Let's say for Vin = 3 VDC and another scenario Vin = 10 VDC. Without any further details, i mean value of resistors and diode forward voltage (i guess 0.7 but still). The thing is how to tell the polarization of those diodes?
I know that if this is diode: A ---->|---- K so it should be polarised in such way: + A ----->|----- K - to allow current flow through it with 0.7 V drop.

Thanks in advance!

 

Sorry for posting in wrong section. I still dont get the way to deal with such circuit. I dont know where to start, you said to remove D1 but why actually this one. I draw some schematics as you typed. But im unfortunately still confused :/.