shooraijin wrote:You might also use it to differentiate, like,
Dare wa Tokyo ni ikimashita ka?
Ruuku ga ...
etc. to point out and emphasize it's Luke that did it.
When using dare, itsu, nani, etc, you must either say 'dare GA anata desu ka' or 'anata WA dare desu ka'. In Japanese, the interrogative word (what, who, etc) usually comes second, unlike english.
as for differences between 'wa' and 'ga', 'wa' is used much like "as for..." in English. So, 'wa' is used to mean 'as for ---- (but not necessarily for anything else)'. In other words, it points out the existence of a specific thing (or qualifier).
for example:
日]
[align=left]
On sundays (but not on other days) I play golf.
'ga', however, is used when pointing out the existence of a nonspecific thing, for example:
うちの前に、猫があります。 uchi no mae ni, neko GA arimasu.
There is a cat (not any specific cat) in front of the house.
You can see how 'wa' points out a specific day, to the probable exclusion of others, but 'ga' indicates just any old cat. There are other differences, but this is probably the most important (and useful).
minasan, ganbatte kudasai!