Formal:
* Bom dia (Good day) - Used in the morning.
* Boa tarde (Good afternoon) - Used in the afternoon.
* Boa noite (Good evening/Good night) - Used in the evening and at night.
Informal:
* Olá (Hello) - The most common way to say hello.
* Oi (Hi) - A very casual greeting.
* E aí? (What's up?) - A more informal way of saying hello, often used among friends.
Other options:
* Saudações (Greetings) - A more formal and less common way to say hello.
* Bom dia, tudo bem? (Good morning, how are you?) - A more polite way to start a conversation.
Tips:
* Bom dia is used from sunrise until noon.
* Boa tarde is used from noon until sunset.
* Boa noite is used from sunset until sunrise.
The best way to choose the right greeting is to consider the situation and your relationship with the person you are speaking to.