to say therefore in other ways, you can use; so, for that reason, consequently, hence, thus , that's why and accordingly. these are used to show result.
but to say however, this is different. you can use; but, despite, in spite of, although and on the other hand. these are used to give opposite information.
e.g.
Sam is a hard working pupil, so /thus/that's why/ consequently/ accordingly/for that reason he always comes first