Why can we see the current source as a open circuit and voltage source as a close circuit?
1.Why can we seem the current source as a open circuit,and voltage source as a close circuit?


Short answer: By definition.

A real voltage source is modeled as an ideal source with small output impedance connected in series. And, a real current source is modeled as an ideal source with large output impedance connected in parallel:



Look at the left-most circuit: If the series resistor has a non-zero resistance then there will be a voltage drop across that resistor when a load resistor is connected across the output terminals (Example: Assume the source is 5V, the series resistance is 1 Ohm and you loaded the source with 5 Ohms. You expect 5V/5 Ohms = 1Amps. But no, the voltage will be divided according to Ohm's law.). That is what happens in practice, actually. But in theory, the series resistor is zero. That's why we replace a voltage source with zero-resistance (i.e. short circuit).

Now look at the right-most circuit. If the parallel resistor has a finite resistance then the current will be divided when a load resistor is connected across the output terminals.


  2.Why can we know when RL=Rth=4, it has the maximal power?


It comes from maximum power transfer equation. It states that the load resistance should be equal to the source resistance. You can prove this by yourself (by solving a simple derivative that equals to zero). In the equivalent circuit, source resistance is, of course, R5 + R6 + R2. 


  3.why is the Rth=1+2+1,not the Rth=1+2+1+5+3 or Rth=5+3 ?


Now you should see why.