* "Olá" (pronounced "oh-la") is the most common and general greeting, similar to "hello" in English. It is appropriate for most situations.
* "Bom dia" (pronounced "bom dee-ah") means "good morning" and is used until noon.
* "Boa tarde" (pronounced "boa tar-dee") means "good afternoon" and is used from noon until sunset.
* "Boa noite" (pronounced "boa noy-tee") means "good evening" and is used after sunset.
You can also use "Ola, boa tarde" (pronounced "oh-la, boa tar-dee") to combine "hello" and "good afternoon," which is a polite way to greet someone in the afternoon.
It's always a good idea to try to use the appropriate greeting based on the time of day.