I'll try to rewrite your question: - If I want to shorten an expression, eg. "I am staying in my room," is "I am in my room now" correct or not? - I am also wondering what the rules are when I use a preposition or preposition phrase. Just give some examples, please. Thank you. I have to agree with ElTempo... there are more important mistakes to fix first. You're also using lazy abbreviations and expecting us to magically understand them. To answer the question about your examples, they are sort of close but I don't think they're the same.

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Your sentence isn't incorrect but it lacks the meaning you're trying to convey. (What you are saying is just a response to "Where are you?" "I'm in my room now") As far as I'm concerned using a preposition to end a sentence may be pressing the matter at hand without revealing or feeling the need to explain every last detail. (That is why you would want to make a sentence shorter. Not that it would be the only reason you could just wish to be brief in response.) If you were arguing through the door with someone you could easily say "I'm staying in my room now!" as in right now you shall continue to stay in your room until undisclosed period of time. Other examples could be; "He hasn't been the same since." - Since what... leave that for later... or perhaps it was explained prior and doesn't need to be again. "We need to get on." - They could be racing for a train, it's not as if you'd be chasing it saying "We better get on train 147C before it leaves the station and we have to wait 45 minutes for the next one." You seem to understand the basic idea of shortening your sentence using prepositions. I hope that helps understand a bit more.

To be honest, I don't really understand your question. It seems you want to make a short sentence even shorter. Why? And I think you should learn other rules, like capital letters, full stops, and commas. Anyway, what's your question?