Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
warn [wɔːn] vt
to warn sb (not) to do/of/thatпредупрежда́ть (предупреди́ть*perf) кого́-н (не) +infin/о +prp/, что
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: