Here are some examples:
Necesitamos un horno que funcione consistentemente.
We need an oven that works consistently.
Quiero que mi fiesta dure cinco horas sola.
I want that my party lasts only five hours.
Mi mamá busca a alguien que pueda limpiar nuestra casa los martes.
My mom is looking for someone who can clean our house on Tuesdays.
Let's take a closer look at them. In the first example, you see that there are two distinct subjects ("we" and "the oven"). There are also two different verbs (necesitar (to need) and funcionar (to work)). The first subject (we) sets up the statement by talking about what they need. The relative pronoun que (that) separates the first half of the sentence from the second half. The verb, funcionar, is describing the type of oven that the first subject (we) needs. It is not known whether they will find such an oven out there in the world, but they are stating what they need in case someone may know of one or to confirm with each other that that is the type of oven they will need in the future. An oven that works consistently may or may not exist. It may be non-existent.
The second example above also has two different subjects ("I" and "my party") and has two different verbs (querer (to want) and durar (to last)). There is also a relative pronoun que (that) separates both verbs and each half of the sentence. The sentence begins with a wish or a desire for something to happen. The second part of the sentence describes what the first subject (I) would like to happen. Please note that this hasn't happened yet, but there is a possibility of it happening in the future. Because this is talking about a future duration for the party, it is considered indefinite. It is not known whether the party will last that long at all. There may be circumstances out of the person's control that would shorten or lengthen the party.
In the final example, there is a person looking for another person to do something specific for them. Again, there are two subjects (mamá (mom) and alguien (someone)) and two verbs (buscar (to look for) and poder (to be able to, can)). Like the other two examples, both parts of the sentence have a relative pronoun que (that) separating them. Now the first subject (mom) may know of someone who can clean the house, but maybe that person cannot clean on Tuesdays. Perhaps she may not know of anybody who can clean her house, much less clean on Tuesdays. Here we are talking about someone who is non-existent at the time the sentence is being spoken.
Of course, if any of these things or people do exist and you can see them, point them out in a crowd, or mention their name, the second verb would be in the indicative rather than the subjunctive because they exist. They are real.
For example:
Mi mamá quiere que Laura limpia nuestra casa los martes.
My mom wants that Laura cleans our house on Tuesdays.
The first subject, mom, has a specific person in mind to clean the house on Tuesdays -- Laura. Mom specifically sees Laura or knows that Laura can clean her house on Tuesdays. Consider this to be a done deal. Laura has already agreed to do the task and will be doing it on Tuesdays. There is no doubt. There is no uncertainty. This is going to happen.
Now if mom is not sure if Laura can clean her house on Tuesdays or clean her house at all, she would use the subjunctive in the second part of the sentence like this:
Mi mamá quiere que Laura limpie nuestra casa los martes.
My mom wants that Laura cleans our house on Tuesdays.
They say the same thing, but the only difference is that there are some doubts as to whether or not Laura will clean the house or can do it. I know, it's a slight difference, but nevertheless, it is a difference that can change the entire sentence and meaning of the sentence. This is one of the reason why the subjunctive "mood" is very important to understand and use properly.
Now it's your turn! In the comments section below, write two sentences talking about indefinite things, events or non-existent people that you or anybody might want, need or desire. Practice using the subjunctive. The more you practice, the better and easier it will get. Get wacky; get creative; get subjunctive!
Espero que haya disfrutado aprendiendo el subjunctivo durante el verano conmigo.
I hope that you have enjoyed learning the subjunctive during the summer with me.
¡Hasta luego!
~Keith