"My grandparents are a little short."
Translation:Mis abuelos son un poco bajos.
I think E3D0A is saying it is preferable or more polite to clarify you mean a person is short by using 'bajitos' where the 'ito' is a form of endearment or by adding estatura (stature) because 'bajos' alone can sound ugly or be interpreted as implying the person is something indecent.
I believe its because when the person or persons are the subject you do not use the personal "a". Use it for direct objects. You have to watch "gustar" though because the "...is pleasing to..." translation makes the person/persons direct objects and therefore the "a" gets used. Please correct me if I am wrong.
I believe your statement about not using the personal 'a' for subjects is correct. However, the 'a' in "A mí me gusta la comida mexicana." (I like Mexican food.) is a preposition. The 'a mí' (to me) is used for emphasis because "Me gusta la comida mexicana." translates as "Mexican food is pleasing to me." --> "I like Mexican food." Using the prepositional phrase can provide clarity when the indirect object pronoun has multiple possible translations. 'Le' can mean 'to him/her/it/you (formal)'.
A usted le gusta la comida mexicana. --> You like Mexican food.
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