No hables como si fuese una persona
[Caption 17, Provócame > Pilot > 5]
If you're a native English speaker, you're likely to translate the phrase above as "Don't speak as if he were a person." Without much thinking about it, most native English speakers choose the subjuntive "were," and not the indicative "was." Ana, similarly, instinctively uses fuese (the subjunctive form) and not era (the indicative) when she tells Mariano "Don't speak as if he were a person". (She is referring to a horse that goes by the name Chocolate.) The subjunctive, as most of us have heard but often fail to fully grasp, is used to express doubt or uncertainty, or to describe situations that are unlikely. Since it is quite "unlikely" that the horse in question is a person, and Ana, sin duda, "doubts" that he is one, she goes with the subjunctive, fuese.
This were/was distinction is one of the few and dwindling instances whereby English speakers retain a subjunctive form (were) that differs from the indicative (was). Other than "to be," most English verbs have melded both the indicative past and the subjunctive past into a single "universal" past tense that encompasses both. For this reason it's often said, somewhat erroneously, that subjunctive tenses "don't exist" anymore in English, and is why English speakers find Spanish's distinct subjunctive tenses difficult to acquire. The more we, as learners, immerse ourselves in authentic spoken Spanish, the faster we too can begin to acquire a native-like "instinct" for the subjunctive and its use.
If you want to bend your brain around the topic further, here are some sites where you can do so: