Expressing Uncertainty, Doubt, Negation

Verbs of doubt and negation require the subjunctive in subordinate clauses; examples include

negar - to deny
dudar - to doubt
no ser verdad - to not be true/the truth
no estar cierto/seguro - to be unsure, uncertain
no creer - to not believe

Remember that expressions of certainty or belief take the indicative:

no negar            - to not deny
no dudar            - to not doubt
afirmar             - to affirm
creer               - to believe
estar cierto/seguro - to be sure, certain

• Dudamos que salgan bien.
  We doubt they'll do well.
• No creo que asistan a la clase.
  I don't think they attend class.
• ¿Niegas que yo pueda hacerlo?
  Do you deny that I can do it?
• No estoy segura de que venga.
  I'm not sure she's coming.

NOTE: Normally the reverse (positive/negative) of each of the above sentences does not indicate doubt or negation and thus takes the indicative.

• No dudamos que salen bien.
  We don't doubt they'll do well.
• Creo que asisten a la clase.
  I think they attend class.
• No niegan que yo puedo hacerlo.
  They don't deny that I can do it.
• Estoy segura de que viene.
  I'm sure she's coming.