as a response to "permesso"? Where permesso means: can I come in? / can I get past? and prego means: please do/please come in:
a. Posso entrare?
as a classical reply to "grazie"
as a way to ask "Can I help you?" (in a shop for instance):
as a way to say "there you are" or "here you are" (when giving something to somebody):