Is buddhism nondualistic? Buddhism learns that everything is in constant change and in constant interaction; that nothing is steady and separated from the other things. Does that mean that buddhism is nondualistic?
Original Pali Buddhism does not teach about 'non-dualism'. Instead, it teaches about 'voidness' (selflessness; sunnata). 


  The world is empty...In what respect is it said that the world is empty? Insofar as it is empty of a self or of anything pertaining to a
  self: Thus it is said, Ananda, that the world is empty.
  
  SN 35.85


In a number of places, it states there are internal &amp; external phenomena.


  The six internal media should be known. The six external media should be known. 
  
  MN 148
  
  Owing to a dyad (pair), monks, consciousness comes into being. And how, monks, does consciousness come into being owing to a dyad?
  
  "Owing to the eye and forms arises eye-consciousness. The eye is impermanent, changing, 'becoming-otherwise.' Forms are impermanent,
  changing, 'becoming-otherwise.' Thus this dyad is fleeting and
  transient; impermanent, changing and 'becoming-otherwise.' That cause,
  that condition, that gives rise to eye-consciousness — that also is
  impermanent, changing, becoming-otherwise.' And how, monks, could eye
  consciousness, having arisen dependent on an impermanent condition,
  become permanent? Now the coming-together, the falling together, the
  meeting-together, of these three things: this, monk, is called
  'eye-contact.' Eye-contact, too, is impermanent, changing,
  'becoming-otherwise.' And how, monks, could eye-contact, having arisen
  dependent on an impermanent condition, become permanent? Contacted,
  monks, one feels. Contacted, one intends. Contacted, one perceives.
  Thus these states also are fleeting and transient; impermanent,
  changing, 'becoming-otherwise.'
  
  SN 35.93


MN 1 lists the perception of 'oneness' ('ekatta') as not Nirvana &amp; as one a many possible experiences. 


  He perceives earth as earth...He perceives water as water... fire as fire... wind as wind ... beings as beings... gods as
  gods...Pajapati as Pajapati...Brahma as Brahma... the luminous gods as
  luminous gods... the gods of refulgent glory as gods of refulgent
  glory... the gods of abundant fruit as the gods of abundant fruit...
  the Conqueror as the Conqueror ... the dimension of the infinitude of
  space as the dimension of the infinitude of space... the dimension of
  the infinitude of consciousness as the dimension of the infinitude of
  consciousness... the dimension of nothingness as the dimension of
  nothingness... the dimension of neither-perception-nor-non-perception
  as the dimension of neither-perception-nor-non-perception[3] ... the
  seen as the seen... the heard as the heard... the sensed as the
  sensed... the cognized as the cognized ... singleness as singleness (ekatta)...
  multiplicity as multiplicity ... the All as the All ... Nirvana as
  Nirvana...


This essay may help: Dhamma and Non-duality.