Magick is a neutral force found in nature, like energy. When someone casts a spell, they call upon this energy, infusing it into their spell, giving this energy a positive or negative charge in the process. the energy is then released into the universe to make the spell work. Since this energy is a force of nature, it is bound to follow the rules of nature. Therefore, anything that cannot naturally occur cannot be accomplished by magick.
Spells to make you physically transform, or grown animal ears, wings, or tail, to name a few, do not work as humans cannot naturally do these things. Also impossible are spells to turn you into an mythical creature, travel physically through time, turn invisible, levitate, fly, and anything else you might of seen in a movie or television. These are things of fantasy to entertain others, not teach.
Spells that work with forces and energies to create subtle changes in ones life do work. Spells for luck, love, wealth, jobs, protection, healing, curses, and my types of daily life will work. It's not as glamorous as Hollywood would make us believe, but it is powerful, and should be used when needed.
Finally, we have our grey areas, spells that some see as impossible, while others see as doable. Study and avoid these spells until you have a better grasp of magick, so you understand better. The easiest and safest of these would be beauty spells. A spell to make you instantly thin and beautiful will not work, but a spell to be incorporated into a daily routine should work over time.
The second is weather spells, with these, you are not creating snow, but moving whole weather patterns into your area to make it snow. This can be a bad thing as you would be effecting the planet just for your desire for a winter wonderland. The spell could cause a flood in one area, a drought in another, or a blizzard where you are. Many see this as too much power for one person, so they see weather magick as impossible for humans to do. Others see it as a force of nature, dealing with energy, so it can be accomplished.
Lastly we have death spells. As with the weather magick above, death spells are seen by some as having too much power for a human to have, while others see it as an extremely powerful curse. Since curses can be cast, a death spell on another would be as well. Same as a weather spell, you should think long and hard before casting one if you have drawn the conclusion it is possible, as you are taking a persons life, negative karma will hit you hard in this case. Do not cast one for fun. casting a spell to see if it works, or to show off rarely works since your own desire isn't there, but if these spells are possible, and you do manage to kill someone, would you be willing to live with the consequences?
To sum up though, before you cast anything, just ask yourself 'does this seem plausible?'