"He works at this farm."
Translation:Ele trabalha nesta fazenda.
They're contractions of "em + esta" and "de + esta", respectively. Since "to work at" is "trabalhar em", only "nesta" would make sense. Other verbs that require "de" (e.g. gostar de - to like) will force you to use "desta" and the like:
- Ele trabalha nesta fazenda. (trabalhar em + esta).
- Ele vive nesta fazenda. (viver em + esta)
- Ele gosta desta fazenda. (gostar de + esta)
- Ele sai desta fazenda. (sair de + esta).
Hope these links help you:
Here you go: https://en.wiktionary.org/wiki/Template:Portuguese_demonstratives (the last column is not important for demonstrative pronouns, only adverbs).
Once you understand the meaning behind each set, it really is just a work of assembling the parts like a Lego :)
An English translation won't do you any good - we have three different demonstratives where English only has two, and Brazilians usually use the middle one "esse" in place of or in tandem with "este", which just complicates things even further.
If you're trying to use the regular demonstratives (the ones that have a noun following them), all you need to know is that the 3 different groups represent three different ideas:
- The "Este" group, the proximal demonstrative, means "this thing next to me, the speaker"
- The "Esse" group, the medial demonstrative, means "that thing next to you, the listener"
- The "Aquele" group, the distal demonstrative, means "that thing over there, away from the both of us".
The rest of the bracket (save for the neuter demonstratives, which are in a category of their own) is formed by treating the demonstrative as an adjective (i.e. making sure it has the same number/gender as the noun it's qualifying); if for some reason "de" or "em" is supposed to show up before the demonstrative, you add d- or n- to the stem, respectively.
Examples (letters in brackets disappear with the contraction):
- d[e] + est[e] + as (feminine plural ending) = destas camisas ("of these shirts")
- [em]n + aquele + s (masculine plural ending) = naqueles castelos ("in those castles [over there]").
-  + ess[e] + a (feminine singular ending) = essa casa ("this house")
Keep in mind that "of" and "in/on/at" are meanings of the prepositions "de" and "em" without any context; when they're attached to verbs they may not carry the same idea (e.g. "gostar de", to like), but they still need to be contracted:
- Eu gosto destas camisas. I like these shirts.
I'm sorry if that was more confusing for you, but it's important you understand the rules in their own environment and that you try to break free from the translations for the ideas to set in naturally :) Good look with your studies!