Learn Polish: Proszę wejść i usiąść. - Please come in and sit down.

In Polish, you can make a polite request or command by using the word proszę (please) followed directly by a verb in its infinitive or dictionary form. It literally translates to 'I ask to come in and to sit down.'

Is there a difference between usiąść and the verb siedzieć?

Yes! The word usiąść means the action of sitting down, or changing your position from standing to sitting, which fits this command perfectly. The word siedzieć means to already be in a sitting position.

What does the ending -ść tell us about the words wejść and usiąść?

The -ść ending is a common verb ending in Polish, just like the more common -ć. It tells you that the verb is in the infinitive or dictionary form.

Could I say this sentence to multiple people, or just one person?

You can use this exact sentence for both! Because using proszę with an infinitive does not specify exactly who you are talking to, it works perfectly whether you are inviting one person or a whole group of people into the room.