"Ren niet weg!"
Translation:Do not run away!
How do you know when to say "ren niet weg" or "Niet wegrennen"? Are they interchangeable or is there a difference in meaning?
Yes, the're interchangeable. I use 'niet wegrennen' more than 'ren niet weg'. But both are correct.
how often are you having to tell somebody not to run away!!! what are you doing!
It's because wegrennen is a separable verb, and when separable verbs take a finite form, the particle is detached from the verb and is placed at the end of the clause.
As in the suggested Dutch version the verb is conjugated, 'Ren niet weg', (and not 'niet wegrennen'), then the particle must be placed at the end of the clause.