If you ever wondered how to warn someone in Serbian, let me give you a list of phrases, starting from the most formal ones:
Formal and semi-formal warnings:

Informal warnings:
  • Pazi se + noun in the Genitive case or gerund (verbal nouns ending in -nje, again in Gen.)
  • Nemoj da + present (2nd person Sg or Pl)
  • Šta god da radiš, nikako nemoj da + present (2nd person Sg. - with 2nd p. Pl, start with Šta god da radite, nikako nemojTE da...) 

If you need all these phrases written both in the Cyrillic and Latin script, pronounced slowly, check  Serbian phrases link.