The subjunctive is used in a subordinate clause when the main clause expresses doubt or uncertainty on the part of the speaker. The expressions below need a que before using the subjunctive.

es posible que - it's possible that
puede ser que - it could be that
es fácil que - it's easy that
es difícil que - it's difficult that
es probable que - it's likely that
es imposible que - it's impossible that

Examples:
Es imposible que los estudiantes se acuesten temprano.
It is impossible for students to go to bed (acostarse) early.

Es fácil que los maestros chismeen.
It is likely that the teachers gossip (chismear).

Es probable que Felicia vote por Cárdenas.
It’s probable that Felicia (will) vote for Cárdenas.

The most common verb used to express doubt is dudar (to doubt).

Dudan que Ana llegue a tiempo.
They doubt Ana will arrive on time.
Dudaban que Ana llegara a tiempo.
They doubted Ana would arrive on time.

Sometimes, doubt is expressed and the subjuctive is not used. This is the case with the following:
creer
parecer que
pensar
No es dudoso

Creemos que ella es bonita.
We think that she is pretty.

Creíamos que ella era bonita.
We thought that she was pretty.

Parece que va a llover.
It seems that it is going to rain.

Parecía que iba a llover.
It seemed that it was going to rain.

Pienso que ella viene ahora.
I think that she comes is coming now.

Pensé que ella venía ahora.
I thought that she was coming now.

The expressions below do NOT need a que before using the subjunctive.

quizás - maybe
tal vez - perhaps
a lo mejor - perhaps
posiblemente - possibly

Quizás tengamos un examen mañana.
Perhaps we have an exam tomorrow.

Tal vez el programa empiece a las cinco.
Maybe the program begins at 5.

Doubt uses subjunctive
Es dudoso que + subjuntivo
No es cierto que + subjuntivo
No creo que + subjuntivo
No pienso que + subjuntivo
No parece que + subjuntivo

Certainty uses indicative
Es cierto que + indicativo
Es verdad que + indicativo
Es evidente que + indicativo
Es obvio que + indicativo
No es dudoso que + indicativo
No dudo que + indicativo
Examples:
Es cierto que Marisa viene hoy.
Es verdad que Julia y Marisa llegan hoy.
Es evidente que estas enojada.
Es obvio de que Juan ama a Julia.
No dudo que tengo que ir a clase.