"Buonanotte" is similar to "Good Night", we use this as a hail when you (or someone else) are going away in the evening (as if you are going to sleep). "buonasera" is a type of hail similar to "good morning" but you use it in the evening (I don't know if you say in english "good evening" but this would be its meaning). You can use it when both you (or someone) arrive and when you go away (like "ciao"), but it is quiet formal "prego/di niente" have a similar meaning (as "you're welcome")but "prego" is a "jolly", you can use it for other things, for exalmple, if you don't understand something you can ask "sorry?" in italian "scusi?" but you can also say "prego?" it's like a permission and a request to you to tall again at the same time. also "spiacente/mi dispiace" are similar, you can use both, but we can say that if it is your fault it's better "mi dispiace" because "mi" is more personal

Buona sera - utilizas al principio de la reunión (en la noche) para saludar a la gente. Buena notte - utilizas al final de la reunión (en la noche) para saludar a la gente antes de irte al dormir (antes de que alguién se va a dormir) Prego - La respuesta al "gracias - grazie" de la manera muy formal di niente - de nada También puedes decir : No hay de qué - Non c'è di che Mi dispiace - lo siento Spiacente - tal y como Patrick te lo dijo, es una manera un poco formal para decir "lo siento" Espero que eso te ayude. Si tienes otras preguntas, puedes preguntarme sin ningún problema. ¡Que tengas un buen día!

buonanotte: it is a farewell in the night. you say that to bid farewell when (you think) that the person is going to sleep buonasera: you use it in the evening to salute a persone/people you meet. example: you are invited by a friend for dinner. when you enter his/her apartment you say:"buonasera". when you leave after dinner you say "buonanotte" prego/di niente have the same meaning. di niente is less formal than prego (but this could be my personal understanding) spiacente/mi dispiace have the same meaning but "spiacente" is more formal. You don't use "mi dispiace" with a person who is not your friend or with whom you have no confidence.

Related to 'prego', please take a look to my answer to this question: http://classic.italki.com/question/355069