Olá, @dbuachalla. Your confusion is perfectly understandable! Since neutral pronouns simply don’t exist for us, for each context where they’d appear in English, you may see a number of possibilities in Portuguese, like you said:
Omissions (as mentioned in this introductory Learning Note - Portuguese Verbs and Personal Pronouns)
- For example, in sentences where it is standing in for an undetermined or inexistent subject:
It rained yesterday. = Choveu ontem.
It is good to talk to you. = É bom falar contigo.
It is so beautiful! = É tão bonito!
Using the omitted noun directly or using other personal pronouns/words contracted with pronouns
- When it is replacing a known being/thing/concept (use a pronoun of the respective noun’s gender):
- Oh, that series! Did you like it? = Oh, essa série! Gostaste da série? = Oh, essa série! Gostaste dela? (3rd Person Possessives: De + Pronoun)
- The dog is large, but it is not aggressive. = O cão é grande, mas ele não é agressivo.
- I like the house. Is it for sale? = Eu gosto da casa. A casa está à venda? = Eu gosto da casa. Ela está à venda?
Using clitic pronouns (Introduction to Clitic Object Pronouns)
- You can fill a glass with water and drink it. = Tu podes encher um copo com água e beber a água. = Tu podes encher um copo com água e bebê-la. (Clitic Pronouns: 3rd Person).
- I love it. = Eu amo-o (Clitic Pronouns: 3rd Person).
- They teach it in school. = Eles ensinam-no na escola. (Clitic Pronouns: 3rd Person)
Using other word classes (if context allows)
- Do it! = Faz isso! (lit., do that)
- We can carry it. = Nós conseguimos carregar aquilo. (lit., we can carry that)
- I don’t like it. = Eu não gosto disto. (lit., I don’t like this) (Combining “De” with Demonstratives)
As you can see, it is not an easy topic to condense because it brings up a lot of related topics. Most/all of the information you need is already on the website, though. I’d say the key is exposure and practice practice practice. It takes time and patience to make sense of aspects like this, where there’s no correspondence at all between English and Portuguese. Feel free to ask any specific questions/go through some more examples, though