1. The Subjunctive mood
The Subjunctive mood is used to indicate that a person does not know for certain if an action is going to take place. It occurs in the 2nd (subordinate) clause, when there is a change in the Subject in the sentence
The four uses of the Subjunctive can be seen in the anagram DIVE

2. D oubt
I nterjection (Impersonal expression)
V olition
E motion

3. Doubt
I doubt that Ana will come to the party.
I don’t think Ana will come to the party.
Perhaps Ana will come to the party.
It is not certain at this time if Ana will be at the party or not.

4. Interjection (Impersonal expression)
If only Ana would come to the party.
Once again, it is not certain at this time if Ana will be at the party or not.

5. Volition
The Subjunctive is also used to show that someone (a 2nd person) wants someone else
(a 3rd person) to do something.
My mother wants Paula to set the table.
Paula told her sister to set the table.
In both cases, it is not certain at this time if the 3rd person will complete the action.

6. Emotion
I hope that Ana will come to the party.
I would like Ana to come to the party.
Once again, it is not certain at this time if Ana will come to the party.

7. I believe that he will arrive on time.
Creo que llegará a tiempo.
I don’t believe (that) he will arrive on time.
No creo que llegue a tiempo.
He won’t work here unless he gets a raise.
No trabajará aquí a menos que obtenga
un aumento.

8. Perhaps she will call us tonight.
Quizás ella nos llamará anoche.
I need an assistant who is punctual.
Necesito un asistente quien sea punctual.
I hope I will get a good grade on the final exam.
Espero que reciba una nota buena en el examen final.