I've understood o far that one uses "das" meaning all of "this/that/these/those" when it refers to some object or thing not yet specified, like "this is cool" or "those are bad".
Let me try better: I'll call this "yet not specified object" YNSO, may I?
First, in the nominative case:
If you wanted to say "this is good jewelry", you would use "das" because you mean something like: - "YNSO" is good jewelry (look: in this case, "good jewelry" is -> qualifying <- the YNSO, by means of the verb 'to be'. Note the relation to the sentences "this is mine" or "that is great!", which do not specify what is "mine", or "great".
Another situation is if you wanted to say "This jewelry is good". In this case, you would be using the word "this" to SPECIFY which jewelry you're talking about, and then in german you wouldn't use "das" anymore, but dieser/dieses/diese (depending on the noun's gender):
- "dieser Schmuck ist gut" ("dieser" for masculine nouns)
- "dieses Mädchen ist gut" ("dieses" or abbreviated "dies" for neuter)
- "diese Frau ist gut" ("diese" for feminine nouns)
- "diese Bäume sind gut" ("diese" also for plural)
But...that's still not our sentence. In the sentence we're discussing: "I like this jewelry", "jewelry" is in the accusative case (when something is being acted on: in our case, liked) , and there is luckly only one change:
- "dieser" becomes "diesen"
Take a look at this wikipedia table (the first two lines) of the demonstrative pronouns https://en.wikipedia.org/wiki/German_pronouns#Demonstrative_pronouns
Hope that helps!
Because "diesen" is the accusative case declination of the word "dieser", wich in turn is translated as "this" or "that". (Look my long answer to "CiarnOConn" above)
On the other hand, the possessive pronoun "your" in the nominative case is translated as: (masc/neut/fem)
- dein / dein / deine (for singular "your", informal);
- euer / euer / eure (for plural "your", informal);
- Ihr / Ihr / Ihre (for formal "your")
And in accusative case, only the masculine ones change, to
great table here: "https://deutsch.lingolia.com/en/grammar/pronouns/possessive-pronouns"
Hope that helps!