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

1. Subjunctive: expressing doubt

a. The subjunctive mood is generally used when the speaker wishes to describe something about which he or she has no knowledge or certainty. For example:

• 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.

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.

2. Subjunctive: making a judgement and expressing emotion

The subjunctive mood is used in a subordinate clause following the expression of an emotion or judgement.

• Qué lastima que tu carro se haya descompuesto.
  It's a shame that your car has broken down.
• Es bueno que el coche tenga aire acondicionado.
  It's good that the car has air conditioning.
• Me enoja que nuestro carro use tanta gasolina.
  It angers me that our car uses so much gas.