Omg~There was a big mouse in my room!

My roommate had often said there were strange sounds in our room at night. She thought that it was a big mouse, but I had never heard or seen it. Last night, I slept at 11 pm as usual. After half an hour my roommate shouted loudly, "Mouse! Big mouse! There and here, oh my god!" she said. I woke up and attacked the big mouse with my roommate, but the mouse was very crafty. We couldn't find it. Finally, we turned on the light and helplessly waited for the mouse to get out of our room. What a terrible night! From now on, we must close the door and window when we go to sleep.

<em>In generall, this is a good narrative piece of writing. Your plot sequence is on point, and your language is descriptive. See some comments below for improvements. Also, it could have been a big mouse or it could have been a rat that you saw. Maybe look up some pictures of both (they are different species) and decide what you actually saw. </em>

*I took out the Omg, because it is not proper in written English. If you wrote this in an academic paper you would get points taken off. Remember that internet lingo should be reserved for social networks or SMS.

**Several of your sentences are run ons. When one idea is finished and another begins you need to use a full stop. If you are continuing the idea and using the comma, you might need a conjuntion or a transition. Examples: conjunction (and, or, but). Transitions (however, finally, although). 

***Remember to use spaces after your commas and full stops.

****Check your verb tenses.

Omg~There was a big mouse in my room

My roommate often said there were strange sounds in our room at night. She thought that it was a big mouse, but I had never heard and seen it. Last night, I slept at 11 pm as usual, but after half an hour my roommate shouted loudly, "a mouse, a big mouse,  there and here, omg!" she said. I woke up and attacked the big mouse with my roommate, but the mouse was very crafty, and we could't find it. At last, we turned on the light and waited
The mouse got out of our room with a blind choice. What a terrible night.
From now on, we must close the door and window when we go to sleep.

<em>[Good work.]</em>

Omg~There was a big mouse rat <em>(mouse and rat are the same word in Chinese - but mouse is small and rat is big)</em> in my room

My roommate often said there were strange sounds in our room at night, she thought that was a big rat mouse, but I was had never heard and or seen it. Last night, I slept at 11 pm as usual, and after half an hour my roommate shouted loudly,"mouse, big mouse, there and here, omg" she said. I woke up and attacked the big mouse/rat, helping my with roommate, but the mouse/rat was very crafty, we couldn't found find it. At last/In the end, we turned on the light and waited for
the mouse got to get out from our room; we had no with helpless choice. What a terrible night.
From now on, we must close the door and window when we go to sleep.