Why does the electric field inside a dielectric decrease when it is placed in an external electric field?

Because charges are induced on the faces of the dielectric which develop a field in the opposite direction in the dielectric . so net electric field inside a dielectric reduces .

draw the graph of electric field with distance from the center of a charged shell of radius

the electric field inside a dielectric decreases due formation of temporary dipole and electric field due to this appose the main electric field. this process is known as polarisation.

The external field induces dipole moment by stretching or re-orienting the molecules of dielectric. Net effect of these induced molecular dipole moment is the production of a field that opposes the external field. So, dielectric gets polarised in opposite direction and, electric field inside dielectric decreases.  

What is the work done in a 2microcoloumb point charge from corner A to corner B of a square ABCD when a 10microcoloumb charge exist at the centre of the square

When a dielectric is placed in an electric field causes polarization of the dielectric inducing a field in the opposite direction. Thus the net electric field inside the dielectric decreases

Because charges are induced on the faces of the dielectric which develop a field in the opposite direction in the dielectric . so net electric field inside a dielectric reduces .

This is Because charges are induced on the faces of the dielectric which develop a field in the opposite direction in the dielectric . so net electric field inside a dielectric reduces .