s/he or singular they?
How to refer to a third person whose gender is to be kept secret in english? Which one is more acceptable or commonly used?

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We usually use "they." I don't know if that's formally correct, but at least in the US, it's what we use.

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If you want the gender to be kept a secret then you would say "they."  However, when writing English correctly, you would never write "they," you would write "he or she."  
The word "they" is not singular, which is why it is not correct to use it when referring to one person....but people get lazy when speaking English and use it like that.

Hope that helps!

Answer

We usually use "they." I don't know if that's formally correct, but at least in the US, it's what we use.