"Proszę" is a very important Polish word and a useful one. We can use it in many situations. Sometimes “proszę“ means in Polish “please”, sometimes- “here you are”, sometimes - “come in!”. You should use the word “proszę“ always when you want to show, that you are a polite person.
Don’t worry about the pronunciation of a typical polish nasal vowel “ę” at the end of the word “proszę“because in this word it sounds just like a simple Polish “e” (you can hear it in a recording inserted at the end of this post). There is a rule, which says that “ę” at the end of Polish verbs sounds always so. It doesn’t mean that Poles never pronounce nasal “ę”, but you don’t need to bother with that in words like “proszę“ . Some Poles try to pronounce “proszę“ with a noticeable nasal “ę” in the end, but the most famous Polish linguists just laugh at them.
How to say in Polish: "Please" & "Here you are"? 
