The first one is about refraction:

when a wave cross a boundary (from a shallow area to a deeper area, why this also regard as two medium?), refraction occurs. Suppose a wave cross from an area of deeper to an area shallower, the wave length as well as the speed decrease, why it must be so (i.e. deeper the area, shorter the wave length is; shallower the area, longer the wave length is), why can't vice versa occurs?

It's about diffraction:

Why the wave direction can be changed during diffraction even it's speed, wave length and frequancy are all still constant?

Each region that has properties affectin the wave is called a medium. In water, depth is one of those properties, and dictates the wavelength.

 

In diffraction: each part of the wave acts as a point source of a circular wavefront. When restricted through an aperture or around a corner, you get behavior of a point source of the part of the wave that can pass by or through.

thats right.

 

thats exatly what happens.

 

to keep it more simple

 

it depends on medieum.

 

think of the medium that wave are travelling through u will get the answer