THIS and THAT - este, ese,aquel
This, that, those, these... they clarify how close or far away something is from the person speaking.
These words are one type of "determiners", words that usually go before the noun, or alone.
The noun can be a person, an object or a non-defined neutral (it, that thing) but the later never exists in the plural.
Let’s see them in order of proximity starting with the closest
THIS - close, here.
Este – masculine
Esta – feminine
Esto – neutral
The neutral can be translated usually as "this thing", and we use it when we're referring to something we're not giving it a name to, not calling it anything, or it's an action.
That means we don't have a noun with gender and number, so we use the default ending, masculine and singular "o".
That's why the -o goes for neutral and we use and "e" for the masculine (este).
Examples of neutral:
Context: You're with some old time friends after not having seen them for a while.
Esto es genial, como en los viejos tiempos - This is great, like old times.
What is "this"? Being with friends, or the situation.
We are not referring to an specific noun (masculine, feminine, singular or plural), so it's neutral.
Estos- masc / Estas- fem
There's no neutral, so the -o goes back to being for the masculine form.
THAT -a bit more far away, there.
It looks the same as the previous but without the "t".
Ese - masculine
Esa - feminine
Eso - neutral
Esos - masculine
Esas - feminine
That over there / back then
You don't have this in English, but it'd be like the previous but more far away, or in the past.
I'd say that if you can translate with "that over there" or "that back then", you can use this one.
Aquel - masculine
Aquella - feminine
Aquello - neutral
Aquellos - masculine
Aquellas - feminine
Example of this third option:
Recuerdo aquel concierto – I remember that concert (far away in time, in the past)
Me gustan estas plantas, pero aquellas son horribles – I like these plants but those (far away, over there) are horrible.
Me gusta mucho ________ cuadro, ¿cuánto cuesta?
I like a lot _____ painting, how much is it?
¿Recuerdas _______ guitarra que compramos en España?
Do you remember ____ guitar we bought in Spain?
¿Ves?, toda ______ ropa es rosa ahora! Tienes que tener cuidado con la lavadora!
See? All _____ clothes are pink now! You have to be careful with the laundry.
¿Qué es ______? Creo que es una mariposa, pero parece una mosca.
What’s ____? I think it’s a butterfly but it looks like a fly.
Tenemos que cambiar _________ cojines (masc) que tenemos en el salón.
We have to change ____ cushions that we have in the living room.
¡________ es imperdonable! No quiero verte nunca más!
¡__ is unforgivable! I don't want to see you anymore!
este/aquella/esta/eso/esos/eso or esto/
Hope that helped! :)