Therefor - therefore

In current English, the two spellings therefor and therefore are distinguished. (They used to be interchangeable.)

• Therefor is old-fashioned, and not much used - except by lawyers. It means 'for that', in many senses of 'for'. "Kings used to name a purpose, and then raise taxes therefor" (i.e. for that purpose); "they could buy something and then pay therefor" (~ for it).

• Therefore is the usual spelling. It has a slightly tighter meaning, derived from 'therefor': it means 'for that reason'.

Therefore is a sentence adverb, and should not be used after a comma (because it isn't a conjunction).