Therefore is a connecting adverb that can be used instead of SO, it is formal.  It can come at the beginning of the clause or after the first word or phrase; or before the main verb.

 

Use therefore for reason + result

(Reason  Result)

• Player A is better than Player B. Therefore, Player A should make more money than Player B.

(Problem  Solution)

I had a headache, therefore I took an aspirin.

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