adjective
1
BAD : Just suppose, for a certain reason, that there was suddenly a shortage of oil.
GOOD : Just suppose, for some reason, that there was suddenly a shortage of oil.
BAD : Every creature must have a certain way of protecting itself.
GOOD : Every creature must have some way of protecting itself.
BAD : He's working in London for a certain travel company.
GOOD : He's working in London for some travel company or other.
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Use certain + noun when you continue (or could continue) by giving details: 'There are certain advantages to living in the countryside, the most important being the fresh air.' 'I'm not allowed to eat certain types of seafood, especially squid and octopus.'
Use some + noun (+ or other ) when you cannot or do not wish to give details: 'In the end, he sold it to some second-hand car dealer.' 'If the factory is shut down for some reason, what will happen to all the workers?'
The phrase some + noun + or other is often used in informal styles to suggest that the person or thing is completely unknown to you and not worth thinking about: 'Apparently, their daughter has got engaged to some shop assistant or other.'
2
BAD : Under some certain circumstances, such as war, food has to be rationed.
GOOD : Under certain circumstances, such as war, food has to be rationed.
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Do not use a determiner (e.g. some, the, their ) before certain when it means 'particular' (see the note at certain 1 above): 'Each member of the committee has certain duties to perform.'