Dic: how, why, comment ?

■ how is it that ... ?  → comment se fait-il que ... + subj ?
■ how come: informal (why is it that) → comment ça se fait

Note: how come is short for the phrase "How did it come to be that..."

• How come you aren't at work? 
• = Tu n'es pas au travail : comment ça se fait ? 
• = Comment ça se fait que tu n'es pas au travail ?

Note:

■ When how is used as a question word meaning in what way? or by what means? (how did you get here?, how will you do it?) it is almost always translated by comment:

• comment es-tu arrivé ici?; 
• comment le feras-tu?

■ When how is used as a conjunction meaning the way in which it is often translated by comment:

• I don't know how they did it = je ne sais pas comment ils l'ont fait
• tell me how you make a curry = dis-moi comment on fait un curry.

■ When how is used as a conjunction meaning that it is almost always translated by que:

• he told me how he had stolen the money = il m'a dit qu'il avait volé l'argent; 
• it's amazing how they survived = c'est étonnant qu'ils aient survécu.

... http://dictionary.reverso.net/english-french/how
http://www.wordreference.com/enfr/how