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Hi, I have a question please
I never see "so" used in writting.
The similar words like therefore, hence, thus, seem too formal.
Is there a word not as colloquial as so but not as formal as the others?
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It not a matter of formality (though we more often say 'so' than 'hence'); it is primarily that 'so' should not usually be used to start a sentence.
Comments  
Thank you for the clarification.
Noted down.