now vs once

now

adv
  • Differently from the immediate past; differently from a more remote past or a possible future; differently from all other times. 

  • Used to indicate a context of urgency. 

  • Sometimes; occasionally. 

  • Used to address a switching side, or sharp change in attitude from before. (In this usage, now is usually emphasized). 

  • At the present time. 

  • Used to introduce a point, a qualification of what has previously been said, a remonstration or a rebuke. 

  • At the time reached within a narration. 

intj
  • Indicates a signal to begin. 

noun
  • The state of not paying attention to the future or the past. 

  • The present time. 

  • A particular instant in time, as perceived at that instant. 

adj
  • Fashionable; popular; up to date; current. 

  • Present; current. 

conj
  • Since, because, in light of the fact; often with that. 

once

adv
  • Formerly; during some period in the past. 

  • Multiplied by one: indicating that a number is multiplied by one. 

  • One and only one time. 

conj
  • As soon as; when; after. 

How often have the words now and once occurred in a corpus of books? (source: Google Ngram Viewer )