Why is real current source represented with resistor parallel with the source, while in the real voltage source resistor is in series with the generator? Can you give me an example for this situations?
Think a bit for yourself. Would a resistor in series with a current source change the current to the load at all? Would a resistor in parallel to a voltage source affect the voltage on the load? The reason for those resistances is to model that the sources are not ideal. In an ideal voltage source, the series resistance is zero. In an ideal current source, the parallel resistance is infinite. In a real voltage source, part of the voltage is wasted in its series resistance. In a real current source, part of the current is wasted going thru its internal resistance (which is in parallel, so that current is 'stolen' from the load).–
If you will convert a voltage source in series with a resistor to current source in parallel with the resistor (and vice versa) both will have same open terminal voltage and will have the exactly the same external current when the terminals are shorted. So you can take any of them interchangeably which suits you according to the problem.