Formal:
* Bom dia: Good morning (used from sunrise to noon)
* Boa tarde: Good afternoon (used from noon to sunset)
* Boa noite: Good evening/good night (used from sunset onwards)
Informal:
* Olá: Hello (used in most situations, can be both formal and informal)
* Oi: Hey (more casual and informal)
* E aí? What's up? (very informal, like "Hey, what's going on?")
Other common greetings:
* Tudo bem? How are you? (literally "Everything good?")
* Tudo bom? How are you? (informal version of "Tudo bem?")
* Como está? How are you? (more formal)
* Como vai? How are you? (informal)
Tips:
* "Bom dia," "Boa tarde," and "Boa noite" are always formal.
* "Olá" and "Oi" can be used in both formal and informal contexts.
* "E aí" is always informal.
* It's always polite to greet someone before starting a conversation.
Remember: In Portuguese, it's important to consider the context and the person you're speaking to when choosing a greeting.