The moon was shimmering like a round, pale coin. Through a window, a single shaft of moon-light shined the gloomy room , where Clarence was resting . Suddenly, he woke up covered with sweat, his heart bumping faster that usual, forced him to breathe with difficult . Extremely confused, he gawked at the outside, realizing that the werewolf’s curse was inevitable: the full moon was lying untouchable in the sky. Immediately, his soft skin was covered by a pitch-black, dense fur, His nails got longer, until they became four powerful, sharp-edge claws. As soon as his fangs appeared, where they used to be pure-white human tooth,. his eyes were stained by a yellow, devilish color. Clarence metamorphosis finished with a incredible and mournful howling, Finally, the instinct of the mythological beast devoured his humanity, completely

Alrighty, that's another good start! I'll do some editing for ya:

Your first sentence is pretty much perfect, good simile for a description.

Here's your second sentence fixed up: "Through a window, a single shaft of moonlight shone into the gloomy room where Clarence was resting." Be careful with those commas again. ^^ Here's the third: "Suddenly, he woke up covered with sweat, [and] his heart [bumped] faster that usual, [causing] him to breathe with [difficulty]" just made a few word changes to make it grammatically correct. Your fourth sentence is good, just make your colon a semi-colon. "inevitable; the full moon" and maybe change the word "lying" to "laying". I think the fifth sentence is a run-on, so I'll fix it up: "Immediately, his soft [human] skin was covered by a dense, pitch-black fur. His nails grew longer...until they became four powerful, sharp-edged claws." I added the word "human" just to emphasize the fact that your character was turning into a beast, but other than that I only made some minor changes just to enhance the flow. Moving on: "Fangs soon appeared where there used to be pure-white human teeth, [and] his eyes [became] stained a yellow, devilish color." mostly just grammar corrections, I changed "were" to "became" since your character is going through a tranformation, and you can add more adjectives to spruce up the imagery if you like. Another piece of advice: you can never have too many adjectives or descriptions. Imagery is a very important part of writing because it lets the reader "see" what's happening in the story, and readers always love very detailed and vivid descriptions. Last part: "Finally, the instinct of the mythological beast devoured his humanity completely. Clarence's metamorphosis finished with an incredible and mournful howling." I switched the two sentences around just so it would flow better and make more sense, and fixed the grammar. Hope that helps! You are free to use my correction sentences in your story.

I really liked it thaks you so much :D