"She is going to ask her this tomorrow."
Translation:Ella le va a preguntar esto mañana.
When "this" is a demonstrative adjective = este/esta/esto take on the gender (or lack of gender) of the noun they are describing. They always come before a noun.
Here "esto" is a pronoun taking the place of clause or sentence and is impersonal/unchangeable.
If it were a pronoun taking the place of a known noun it would take on that noun's gender and number.
N.B. Spanish has three demonstrative pronouns:
this (one) /este
that (one) / ese
that (one over there) / aquel
All but the impersonal forms (esto/eso/aquello) take on gender and number.
So, a couple of examples using "that":
"Este coche es mío, pero ese es suyo"
"Esta mesa es mía, pero esa es suya."
For a good rundown of demonstrative adjectives and pronouns see:
Good rundown, luluarosa! Your link answered my question, I think: "Each demonstrative pronoun also has a neuter form. They do not change for number or gender, and they are used to refer to abstract ideas, or to an unknown object." So, if in context you're referring, say, to a book, you would use "este," not "esto." Sorry if this response is a bit out of sequence, but there was no "reply" link to your last input. Edit, now there is!