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.

The subjunctive mood has many uses in Spanish. Two of those uses are to express doubt and to make a subjective judgement.

No creo que el carro tenga ventanas eléctricas.
I don't think that the car has electric windows.

Es posible que el precio incluya servicio de un año.
It's possible that the price includes one year's service.

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.

Doubt--Disbelief

Students should recognize sentences in which "doubt" is expressed. In addition to the word "doubt" itself, there are many other ways of expressing disbelief, such as "I don't believe", "It doesn't seem", "It's unlikely", "I can't imagine", etc. Students should remember that in a sentence in which disbelief is expressed, it is necessary to use the subjunctive form of the verb in the "dependent clause" (not the main verb, except in some very rare cases which we will not go into here).

• (fact) Juan estudia mucho.
• (doubt) Yo no creo que Juan estudie mucho.

Sometimes we express disbelief in an indirect way. Rather than saying, "I don't believe" (or anyone specific), we make a general statement.

• Es imposible que Juan estudie.
• No es probable que Juan estudie.
• Es difícil que Juan estudie.
• (It's unlikely that John is studying.)

b. On the other hand, when you use expressions of certainty (e.g. Es verdad, es seguro, es cierto etc.) you will use the indicative mood.

• No hay duda que el carro tiene cuatro puertas.
  There is no doubt that the car has four doors.
• Es verdad que ese carro tiene mejor rendimiento de gasolina.
  It is true that car gets better gas mileage.