[Wasabista]

On ni and [b]de[b/]: You use "ni" to mean general location in/at/on for verbs of stasis, but use "de," which also means "with/using," for verbs of action (both transitive and intransitive). Thus:

Watashi wa Saitama ni sunde imasu.
I live in Saitama.

Watashi wa nihongo de hanashite imasu.
I'm speaking Japanese.

Watashi wa do:ro de aruite imasu.
I'm walking on the street.

It's actually a fascinating topic, especially the topic particle wa. Do look for a good grammar book.