Question about German adjective endings?

for plural nouns, the forms of "mein" are:

1. meine -- nominative and accusative cases (and for THE, it's die)

2. meinen - dative case (and for THE, den. Plural nouns in the dative also add an -en or -n, if the plural doesn't form doesn't already end in of them).

3. meiner --genitive case (and for THE, der).

For singular nouns:

masculine: mein or der (nominative); meinen or den (accusative); meinem or dem (dative) & meines or des (genitive, and nouns add -s or -es if they don't already end in them).

neuter: mein or das (nom & acc); meinem or dem (dat) and meines or des (gen. add -s or -es if needed).

feminine: meine or die (nom & acc); meiner or der (dat & gen).

Determiners (der, mein, ein, kein, welcher, etc) decline differently than attribute adjectives (schön).

Attributive adjectives have three declension patterns:

1. strong (when there is no determiner). Schöne Pferden - Beautiful horses.

2. weak (when the determiner is a word for THE, or follows the der etc pattern) Die schönen Pferden -- the beautiful horses

3. mixed (when the determiner is ein, kein, or a possessive determiner like mein). meine schönen Pferden - my beautiful horses.

Basically, -e & -en are the only weak endings, because that type of determiner (like der, die, das etc) gives distinct information.

When there is no determiner, the adjective has to provide the info that usually comes from the determiner, so additional endings (strong declension) are used as necessary: -er, -es, -em, along with -e & en)

When it's ein etc, it's mostly -e & -en, except in a few places, which take the strong endings (mixed declension).

schönen - is plural in the nominative case when it's the weak or mixed declension. Mixed is the one that is used here because of "meine".

It's "meine" because when a noun is plural, only the adjective that follows the article takes an -n, but the article itself doesn't take -n. "meinen" would indicate that it's either a singular masculine noun direct object (accusative case), or plural (masculine, feminine or neuter) in the dative case ("mit meinen schönen Pferden"). (since "mit" is one of the prepositions that requires the dative case after it)

According to Google translate, Das sind meine means "These are mine" and Das sind meinen means "that's mine". With "Pferde" added, they translate the same. German is a very weird language. The articles are declined, and the verb is often at the end of the sentence.