please

there are several forms of the word please in Italian.  Is there a way to know which to use?

Hi David,

There are three forms that come to my mind now: per piacere, per favore and per cortesia. All three are perfectly interchangeable.
Piacere means pleasure ("for pleasure").
Favore means favour ("for favour").
Cortesia means courtesy ("for courtesy").

Are these the forms you were thinking about?

Lucia

Is si prega commenly used?

Yes, but only in formal instructions:

Si prega di non fumare.
Please do not smoke.

Si prega di allacciare le cinture.
Please fasten your seatbelt.

Si prega di notare che...
Please note that...

Si prega can also only be used at the beginning of the sentence and its literal translation into English would be "One begs (you) to...". In other situations, such as in Open the window, please, you would use one of the three translations above: Apri la finestra, per favore/piacere/cortesia.

Hope this helps!