Your confusion is not uncommon. Students often have a problem with this. My suggestion is to replace the ac input with a dc input which you can swap in polarity. Draw it with the dc battery supply one way round and trace the dc current flow, then do the same with the battery supply the other way round. AC is just DC that "swaps over" regularly. I find most of my students "get it" after this.
If the source is an AC source as given in your circuit, the two diodes that are off won't remain off forever. As soon as the source start to be in its negative part of the cycle the ones that are off will turn on and the ones that are on will turn off. Interestingly, the current flowing through the load never changes its direction. That way you will have only positive currents even input voltages are negative.
I know this can get you more confused, but it's worth knowing it. Good luck!!!
In a bridge rectifier (full wave) why do the electrons not flow from the other two diodes during a half cycle? Pls help me in understanding the concept.
First you have to understand the fundamental operation of a diode: it only conducts in one direction.
In a full wave rectifier, there are two diode that conduct when the ac signal goes in a positive direction and the other two diodes are reversed bias. Then when the signal goes negative, the other two diodes conduct while the first pair goes into a reverse bias state (stops conducting).