Doubt, Certainty

In the Spanish language there are you use different forms of words depending on whether or not you are expressing doubt, disbelief and denial or expressing certainty. When expressing doubt, disbelief or denial and if its an ar verb then you add e, es, e, emos, en. If it is a ir or and er verb then you add a, as, a, amos, an. If you are expressing certainty then you conjugate the verb normally.

Dudo que usted va a estudies.
Estoy seguro de que van a estudiar.
Niego que comie los dulces.