One aspect for using the subjunctive is expressing wishes, desires, hopes, and dreams. You would not use
the indicative because that is grounded in reality and certainty. Wishes and the like are part of the uncertain and unreal because they have not happened yet and there is a chance they will never happen. I by no means intend to be a buzz-kill, but where there are wishes, there is improbability -- thus the subjunctive should be used.
Some verbs that express wishes, desires, hopes and dreams are: desear (to desire), querer (to want), esperar, (to hope), buscar (to look for), pedir (to request, to ask for), and agradecer (to be grateful for). Of course there are many more out there that you may use, but these are some common ones that will trigger the use of the subjunctive.
To begin, one of the "wish" verbs must be conjugated to the subject that is doing the wishing, wanting or desiring. Next, a relative pronoun such as que (that), como (how), donde (wherever), quien (who), cuando (when), cuyo(s) (whose), and cuya(s) (whose). Relative pronouns will explain a bit more about a person, place or thing. For example, "a person who knows mathematics" has the relative pronoun of "who" because what followed it gave more information about the person.
Finally, the second half of the sentence may have another subject or it may use the same subject. The second subject is for the person, place or thing that is being wished for to provide additional information about it. That second verb will use the subjunctive.
Let's bring this all together with some examples:
Yo busco a una persona
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quien
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concine comida española.
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Nosotros deseamos una casa
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que
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tenga una piscina.
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Ellas quieren parar
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donde
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puedan.
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Tú esperas
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que
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nos casemos pronto.
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The first example in the chart above says that I am looking for a person who cooks Spanish food. The reason we use the subjunctive is because there is uncertainty as to whether or not such a person will be found. Obviously there are people out there who cook Spanish food, but the question is whether or not the subject "I" will find him or her.
In the second example, it says that we wish/desire a house that has a pool. Like the first example, obviously there are homes that have pools; however, we are unsure that we will be able to find one. Because of the doubt, we use the subjunctive.
For the third example in the chart, I tricked you in that the subject for both verbs is the same subject. The example reads: They want to stop wherever they can. The uncertainty lies with whether or not they can stop at all. Perhaps they are dependent upon a particular mode of transportation or other people taking them as to when they will be able to stop. They might be at the whim of others. Their preference is to be able to stop when they choose, but can they? That is the ultimate question and why we use the subjunctive for that second verb.
Finally, in the last example which reads: You hope that we marry soon, casts doubt onto whether that marriage will happen soon or more in the future. Who knows what history the couple has had in the past. Maybe there is a chance they won't marry at all. Never the less, it is still a desire of one of the two people, but that second verb uses the subjunctive to convey the uncertainty because it hasn't happened yet. It is not because one of the people has doubts about the marriage (though that could be the case), but you might be reading a bit much into it.
To sum up this use of the subjunctive, whenever you are talking about wishes, desires, wants, etc. and you are discussing that wish, desire or want, you must use the subjunctive. Even when you are talking about wishes in the past, you would use the subjunctive to convey that doubt or uncertainty about the reality of the wish. So with that, espero que le vaya bien.(I hope that you travel well.)
¡Hasta luego!
~Keith