Keywords: falsehood, not true, false, uncertain, unsure, untrue
As a Noun
Something which is not true and not in accordance with the facts. English = falsehood.
As a Verb or Adjective
Not to be based on facts or on things that really have happened but on things which are invented. English = (be) not true, (be) not factual, (be) false.
To have doubts that something is in agreement with the facts, to have doubts that something is true. English = (be) uncertain, (be) unsure.
Used alone to deny what someone has said, just short of calling them a liar. English = 'Definitely not!', 'That's not true!', 'Surely not!', etc.