Aquella vs Estos
When to use Aqulla vs estos algunas vs unas and so on, its a bit confusing
Traditionall, Spanish has three basic grades of distance whereas English has 2 basic grades:
esto (this) > eso (that [near]) > aquello (that [far])
aquí/acá (here) > ahí (there [near]) > allí/allá (there [far])
I'm not sure about other varieties, but Chilean Spanish (and my local dialect, not Chilean but from Chile) has only 2 grades: esto > eso and aquí/acá > ahí/allá[/allí].
"Unas" is "some" ("unas hojas" = "some pages") "algunas" is "some" but considering a bigger group, both are used with feminine words. "Tengo unas amigas en esa ciudad" (I have some friends in that city [speaking about that city or about my friends) vs "Tengo algunas amigas en esa ciudad" (I have some friends in that city [I have more friends outside there]).
Aquella is translated that, and it actually refers to something that is not near to either the speaker or the one spoken to. Also, it is feminine and modifies feminine nouns.
Estos is translated these, and it means just that. Also, it is masculine and plural.
Algunas and unas mean the same thing but are typically used in different situations. I’ve noticed that unas is used more often when una would have been used for the singular feminine noun. (For example, una casa becomes unas casas).
Here is a list of demonstratives and articles with their translations:
Aquel, aquella—tha (over there) Aquellos, aquella—those (over there) Ese, esa—that Esos, esas—those Este, esta—this Estos, estas—these Un, una—a, an Unos, unas—some Algún, alguna—any Algunos, algunas—some