Addressing people with -san.

When do you add -san at the end of a person's last name. If last name is Momii, when is it proper to use Momii-san?

Thanks!

The honorific "-san" is quite common and very useful. You can use it either with given name or family name.
You can use it in almost all the situations, to be polite. (But not toooo polite so you can use it when addressing to a friend.)

The only thing you should be aware is that you cannot use it for yourself. Unlike Mr/Mrs/Ms/Miss, calling oneself with "-san" sounds odd and inappropriate. ;-)

As "Magonote" already mentioned you can - NO - you should use it all the time when talking to a Japanese person.

"-Kun" for boys and "-chan" for girls is also very common.

In addition you have the very formal "-samma" - mostly used in letters, however.

Hope this helps a little.

Greetings from Shikoku,

Klaus