Hello Alex! It's hard to talk about rules for "PARA"/"POR" because they are really complex and full of exceptions. I hope this summary can help you: >> MAIN USES FOR "POR" ( and its contractions "PELO"/"PELA") << - Indicating the place where you're going: "ALEX ANDOU POR ESTA RUA" (Alex walked in this street) - Indicating means: "ALEX SEGUROU A CRIANÇA PELO BRAÇO." (Alex grabbed the child by the arm) - Idea of frequency: "ALEX TOMA BANHO DUAS VEZES POR DIA." (Alex takes shower twice a day) - Idea of division, distribution: "ALEX DIVIDIU O PÃO POR DOIS." (Alex divided the bread in two) - Idea of exchange: "ALEX TROCOU UMA NOTA DE R$100 POR DUAS DE R$50." (Alex exchanged a R$100 bill for two R$50 bills) - Indicating cause: "A CRIANÇA CHOROU POR CIÚME." (The child cried for jealousy) - Idea of time: "ALEX FICOU PARADO AQUI POR HORAS." (Alex was paused here for hours) - Passive voice agent: "O RATO FOI MORTO PELO GATO" (The mouse was killed by the cat) >> MAIN USES FOR "PARA" << - Indicating recipient: "ESSES DOCUMENTOS SÃO PARA VOCÊ." (These papers are for you) - Indicating purpose: "É UM CARRO BOM PARA USAR NA CIDADE." (It's a good car for use in the city) - Indicating a destination: "ESTE TREM VAI PARA O RIO DE JANEIRO." (This train goes to Rio de Janeiro) - Indicating time limit: "TENHO CONSULTA MÉDICA MARCADA PARA AMANHÃ." (I have medical consultation schedule for tomorrow) - Indicating personal opinion: "PARA MIM, FALAR SOBRE 'POR' E 'PARA' É DIFÍCIL." (To me, talk about POR and PARA is hard) ------------ I translated my answer for this question (here is the link in the case you think is useful compare the translation with original portuguese): https://www.italki.com/question/272780

This question is complicated to explain, but we usually use this form: PARA When you use PARA, you look forward as an aim. You reply to the questions: What do you do that for? What is the PURPOSE of that? TOWARDS what end? 1. As direction or destination, you'd likely use the English words "TO" or "TOWARDS", e.g. "O comboio para Lisboa é às 18.00 horas." - "The train to Lisbon is at 18.00 hours." or "Eu vou para o norte." - "I'm going towards north". 2. As the purpose of something, you'd use the English words "FOR" "TO" or "IN ORDER TO", e.g. "Para quê faz dieta?" - "what do you diet for?" In short, for what PURPOSE and towards what objective? or "Para passar no exame, preciso de estudar" - In order for me to pass the exam, I need to study". FOR: 1. Is used as FOR (in English) when you look backward as the reason why. You'd reply to the questions: Why do you do that? What is the reason? On the behalf of who? e.g. "Eu telefono ao Pedro por ti." - "I call Pedro for you" = ON BEHALF OF WHO = WHO AS THE REASON WHY. 2. When referring to numbers - For how many years, kilometres, etc.? For how much? e.g. "Comprei esta camisa por 30 dólares" - I bought this shirt for 30 Dollars = FOR + NUMBER. 3. When expressing movement you would reply to the questions: Through where? Via where? e.g. "Eu passo-te o livro pela janela." - "I'll pass you the book through the window." = THROUGH in movement = VIA. 4. When expressing the agent of a passive action replying to the questions: By whom? e.g. "A fruta foi comida por ele" - "The fruit was eaten by him." FOR AS "BY" = THE AGENT OF THE PASSIVE ACTION.

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