preposition
1
BAD : The new airport makes it easy to go outside the country.
GOOD : The new airport makes it easy to get out of the country.
BAD : I'll be going outside London for a few days.
GOOD : I'll be going out of London for a few days.
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When you mean 'away from', use (get/go) out of (NOT outside ): 'It does you good to get out of the city now and again.' 'Without a false passport, he would never have been able to get out of the country.'
2
BAD : She was listening outside of the door.
GOOD : She was listening outside the door.
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In British English, outside is not used with of : 'You aren't allowed to park outside the bank.'
In American English, both outside and outside of are used.