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Hey guys, this is not an easy topic, so I would suggest visiting this url (http://www.sonia-portuguese.com/language/grammar/pronouns/#Demonstrative Pronouns) to make things clearer ;)
"Aquilo" (naquilo as well) refer to indeterminate objects. It's a genderless pronoun used to replace a noun. Think of it as "that thing", instead of just "that". Aquele and aquela are respectively masculine and feminine demonstrative pronouns, and they need an object (aquele livro, aquela casa).
It's one of the idiomatic differences between English and Portuguese (this happens in Spanish as well); pensar + em is to think of or to think about, but it doesn't specifically use the word "from." The best way I can say to help out (what I've seen which has helped me) is to not focus on the direct meaning of each preposition for a certain phrase but rather which goes with each phrase or sentence. In this case, you'd use em (sobre also works tehcnically)