Gender of objective pronouns and demonstratives with no explicit object

Question

Do the words lo, la, los, las (objective pronouns) and este, esta, estos, estas, ese, esa, esos, esas (demonstratives) only have to agree with gender when the word is actually used? What if the pronouns/demonstratives only refer to objects whose words are never spoken? I'll give some examples.

On Direct and Indirect Object Pronouns in Spanish, I saw the following example sentence:

No quiero decírtela. _{I do not want to tell it to you}

In this sentence, we don't know to what word 'la' refers since it is never mentioned, so it would make sense (to me) that 'lo' is used instead (since, for example, 'el' is used instead of 'la' when gender is ambiguous or mixed).

Also, when using demonstratives to point to objects, do you have to make sure the demonstrative agrees with that object, even if you never mention the object by name? If you do have to make the demonstrative agree with the gender, what about ambiguous cases when there are multiple words that refer to the object but have a different gender?

For example, if I point at an apple and say "Look at that" in Spanish, but never say the word "manzana" (or any other words that could refer to the apple, such as "objetivo," which is masculine), what demonstrative do I use?

Part of the reason this is a problem with me, is that making gender agree requires mental effort, especially since I associate gender with words, not actual objects.

Answer 1

Do pronouns only have to agree with their object's gender when the word is actually used?

Well... yes and no.

In the first example you gave, it'd be perfectly fine for you to say to me:

—No quiero decírtelo.

But you can say that because I don't (yet) know what is it you don't want to tell me. You can say "No quiero decírtelo" only as long as "la verdad" is not mentioned anywhere in the previous sentences. But once it is made clear that we are talking about "la verdad", which has feminine gender, it wouldn't make sense to switch to neutral gender again in the next sentence:

—Dime la verdad.
—No quiero Ⓧdecírtelo.

If you change the gender it looks like you're talking about a different thing. So it needs to be:

—Dime la verdad.
—No quiero decírtela.

Similarly, in your second example you can say:

—Mira eso.

And it'd be fine because I don't know what eso is, yet. But once it is clear that it refers to "la manzana", then you need to drop the neutral:

—Mira eso.
—¿La manzana?
—Sí, mírala bien. Es la única amarilla de todo el árbol.

Else it would be really difficult to establish the so much needed context references which Spanish conversation usually relies on.