Why are some constellations only visable from a particular hemisphere ?

Have a look at the angle of the earth to the eclyptic and the motion of the earth round the sun and then you'll understand why some stars are only visible from certain positions. A good atlas should help explain this and I'm sure many internet sites will have suitable diagrams.

At any given moment, you are in a position to see half of the sky. Although the land you are standing on is not truly flat, for practical purposes you can think of it as flat, i.e., as if you are standing on an infinitely large flat surface. All stars that are above that imaginary flat surface are visible to you (assuming that it is night time and the skies are clear).

As the earth turns, the part of the sky you see gradually changes. So in the course of 24 hours, you get to see more than half of the sky. As the earth turns, some stars are disappearing below the western horizon (i.e., they are setting), while others are coming into view above the eastern horizon (they are rising).

But when our own star (the sun) comes up, you will not be able to see any stars until the sun goes down again. So stars that are near the sun in the sky will not be visible, as they are not above the horizon except when the sun is in sight.

However, in the course of a year, as the earth revolves around the sun, the stars that appear to be near the sun keep changing. So if you can't see a star in March, you will be able to see it in a few months, as we move around the sun and the sun is no longer in a line between us and that star.

There are two places on earth where the stars that are visible do NOT change in the course of 24 hours: the North and South Poles. If you stand at the North Pole, the North Star appears to be directly overhead, and all the other visible stars appear to revolve about it. (Of course, they DON'T revolve about the North Star; instead, the observer standing at the North Pole is rotating 360 degrees every 24 hours, which makes the stars appear to travel in circles.) But the point is that no stars rise or set when viewed from the North Pole. The visible stars consist of all stars in the Northern Hemisphere, and that doesn't change as the earth rotates or as the seasons change.

At the South Pole, all of the stars in the Southern Hemisphere are visible (provided it's dark and the skies are clear). So observers at the North and South Poles are each able to see half of the stars, and together they can see all of the stars (but each sees stars only during his own 6 months of darkness).

At any other place on earth, some Northern Hemisphere stars and some Southern Hemisphere stars are visible at any moment, and the view keeps changing (as described above). Observers in the Northern Hemisphere see more of the Northern Hemisphere stars and fewer of the Southern Hemisphere stars; and there are some Southern Hemisphere stars that are too far south for them to see. The opposite is true in the Southern Hemisphere. And for those at the equator, the view of the sky consists of half Northern Hemisphere and half Southern Hemisphere. And in the course of 24 hours (ignoring the fact that stars are not visible during the day), an observer at the equator is able to see ALL of the stars in both hemispheres (except for those that are currently near the sun).

Hope that helps you make sense of how your view of the stars changes over time.

you can't see through the Earth. So you wont see the North star from the south pole. What you can see depends on where you are between the north and south poles. Seasonal changes are which side of the sun you are on. You cant see stars in the day time. At night the stars you see in winter would be daytime stars in summer.

Well the reason just may that the earth is spherical.  Also the position of the earth in it's orbit around the sun!

from the equator you can see them but not at the same time. the line of site horizon ditermans what you get to see.