Hello, how have you been? Read this article, it explain very well: https://www.italki.com/article/586/Tips-To-Use-Definite-And-Indefinite-Articles-In-Portuguese You can also read the rest of the articles, maybe they will help you.

Hello, sorry about my english. We use "no" when we talk about places, like buildings, or areas but with the male gender like "no cinema" it's "in the theatre" . The expression "em" usually when we talk about names of places but cities, villages, towns "em Lisboa" it means "at Lisbon". We also use "em" to talk about time, a specific date "em Janeiro de 1990" it means "on January 1990" . The "no" ou "na" we use to talk about spring, winter, summer, nothing too specific. Did you understand me? :) I hope I helped :)

Witaj! It's about your question about 'no' and 'em'. Em is the "original" preposition to indicate place. Though, "no" and "na" are more specific because they agree with words' genders. Explaining: em + a = na // em + o = no. Then you use "em" when you don't need to use an article right after it. Example: O livro está EM cima da mesa. (The book is on the table). When you need to use an article after the preposition, you must use no or na. Example: Eles falam polandês na Polônia. (They speak Polish in Poland.) // Eles falam português no Brasil. (They speak Portuguese in Brazil). Understand: In English, they dont have genders for words, but in Portuguese, German, French etc, they do. The article (a, o) indicate the gender. PS: It's a short answer that might help you for now, but it's important that you take some time to study prepositions, in any language you're studying.

"Em" is a preposition that can indicate a number of things, such as location or time and some links between verbs and correspondent expressions. It is often translated into English as "in", "at" or "on" among other possibilities. "No" is the contraction of the preposition "em" and the singular masculine definite article ("o"). Other contractions are: "nos" (em + os), "na" (em + a), "nas" (em + as), besides those with other words than articles (for ex.: "nesse" [em + esse] etc.). For you to know when to use "no", you need to have gotten familiar with two things: a) when a noun requires or not the definite article; b) when the preposition "em" can or should be used. It takes some time to learn that well. One of the key recommendations is to practice and try to learn authentic examples.