Good Morning
"Good morning" is both a formal and informal greeting.
Co-workers often greet each other by saying good morning.
Americans say good morning from sunrise until noon.
Good Afternoon
"Good afternoon" is both a formal and informal greeting.
You can say good afternoon to both strangers and friends.
Americans say good afternoon from noon until about 5:00 PM.
Good Evening
"Good evening" is both a formal and informal greeting.
You can say this to both strangers and friends.
Americans say good evening from about 5:00 PM until sunrise. After sunrise, they say good morning.