” The perfect place to start greeting Portuguese is with greetings — learn these and you'll be able to say hello and goodbye to the Portuguese speakers you meet. Below are some of the most common words used for greetings:   Hello: Olá (oh-la) Hi or Hey: Oi (oy) — informal Goodbye: Adeus (ah-deuzh) Bye: Tchau (cha-oh) — informal Note that some of these words are marked with the label "informal." In Portuguese, it can be considered rude to use informal language with people you don't know, people who are older than you, and people in positions of authority. These aren't curse words — just words that aren't very dignified. A good rule of thumb is: don't use informal words with people you aren't close buddies with. Just like English, there are many different ways to greet people in Portuguese. These greetings allow you to remark on the time of day when you say hello:  Good morning: Bom dia (Boh-n dih-ahor Boh-n djih-ah in Brazil) — actually means "good day," but it´s used mostly before noon or before the noon meal. Good afternoon: Boa tarde (Boh-ah tahr-jia) — used after noon or after the midday meal until twilight. Good evening or good night: Boa noite (Boh-ah no-ee-tay) — used from twilight until the morning. Portuguese is no different than most other languages — after you say hello to someone, it's common to ask how they're doing. Use these easy phrases to ask about the people you meet:  How are you?: Como está? (Coh-moh esh-tah? or Coh-moh es-tah? in Brazil) How's it going?: Como vai? ("Coh-moh vye?" (rhymes with "eye")) — informal What's up? (Only in Brazil): E aí? (E-aye (pronounced as one syllable)) — informal Is all well?: Tudo bem? ("Too-doo beng?") — informal Once you ask someone how they're doing, odds are that, at the very least, they'll ask the same to you. Use these responses to tell people who you are and how you're doing:  Well / very well: Bem / muito bem (Baing / moo-ee-toh baing) Bad / very bad: Mal / muito mal (Mao / moo-ee-toh mao) More or less / so-so: Mais ou menos (Ma-eece oh meh-nos) My...

TL;DR in Vietnamese:
Học cách nói "Xin chào! Học cách chào hỏi theo từng thời điểm trong ngày. Học cách hỏi thăm người khác. Học cách nói về bản thân.