While learning Vietnamese on this site, I learned that the Vietnamese word for “therefore” is:
nên
See: http://www.memrise.com/course/260893/beginners-vietnamese/8/
However, I have now learnt that “nên” can also mean “should”.
e.g. you should never play with peoples feelings - Bạn không bao giờ nên chơi đùa với cảm xúc của người
of course, “nên” could be used to mean “therefore” as in:
Cái này rất nóng nên đừng có chạm vào nó - this is really hot, so (therefore) don’t touch it