conjunction
1
BAD : Bring something to eat unless you get hungry.
GOOD : Bring something to eat in case you get hungry.
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When you are talking about something that is done as a precaution, use in case (NOT unless ): 'You should take a book with you in case you have to wait.'
2
BAD : We can go in my car unless if you prefer to walk.
GOOD : We can go in my car unless you prefer to walk.
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Unless (= if ... not) is NOT used with if : 'That's all for today unless anyone has any questions.' 'Don't say anything unless you have to.'
3
BAD : He won't go to sleep unless you will tell him a story.
GOOD : He won't go to sleep unless you tell him story.