Spanish agreement has nothing to do with opinions; it’s a grammatical function of the language. Just as verbs change to agree with their subjects in most languages (“I am” vs “you are”), so too do adjectives and pronouns in Spanish: they change to agree in gender and number with whatever word they modify or replace (un hombre alto vs una mujer alta).
One of the eight parts of speech, adjectives are a type of modifier; that is, they modify or describe nouns in a certain way, letting you know the size, shape, weight, color, nationality, or any of a myriad other possible qualities of nouns.
The Spanish definite article has to agree in gender and number with the noun it modifies, and it doesn’t always correspond to an article in other languages.
Demonstrative adjectives (this, that) are used to indicate a specific noun or nouns. In Spanish, they must agree with the noun(s) in gender and number, and there are 3 different sets: este, ese, aquel.
Demonstrative pronouns (this one, that one, the one[s], these, those) refer to a previously-mentioned noun in a sentence. Spanish demonstrative pronouns are more complicated than their English counterparts, because there are three different sets and because they must agree in gender and number with the noun they replace.
The singular indefinite articles in Spanish correspond to “a, an, one” in English. The plurals correspond to “some.” There are four Spanish indefinite articles.
Every country and continent has an adjective that can be used to describe people and things from that place. Those same adjectives can also be used as nouns to refer to people from that place.