When yo want to great someone, you have to great them based on the time of the day.

in the morning you will say GOOD MORNING

in the afternoon you will say GOOD AFTERNOON 

in the evening you will say GOOD EVENING 

at night you will say GOOD NIGHT 

What are you doing during the time, for example: 

I go to school in the morning

I have lunch in the afternoon 

I do my homework in the evening 

I go to bed at night

now, see the picture below and write the right time for the activities