"She orders him to write."
Translation:Zij beveelt hem om te schrijven.
Without 'om' is correct as well. But when you use 'om' you make the sentence more complete.
Could you tell us WHY it's optional? I.e., is there any case where it would be mandatory?
Actually, in formal, written Dutch it's better to leave out 'om'. Then you should only use 'om' when it's used to indicate a goal, for example: ik gebruik een pen om te schrijven/Ik doe dit om je te helpen... In those 'goal' cases it's always mandatory to write 'om', also in informal Dutch.
to order someone to do something, shouldn't "commanderen" also be accepted as a verb here?
Maybe she is his commanding officer, hence she can order him to do things without it being incredibly rude.
My translation was accepted without "om" but I don't understand why.
"Zij beveelt hem te schrijven."
Does that mean "om" is usually optional?
no, because the Direct Object (hem), needs to come directly after the conjugated verb.
It should also be "schrijven".
I think 'bestellen' means to place an order where as bevellen (?) has the other meaning of order which is giving instrictions that one must follow so the meaning would change and the sentance wouldnt make sense. She is not ordering him in that way in this sentance (unless he is for sale?)