Last week I got asked this question by someone I know and I didn't know how to answer. They were supposed to translate a sentence that goes: "I'm in Japan now" from English to Japanese. There was no context given. The question was:

Why is 私は今日本にいます correct and 私が今日本にいます incorrect, if we do say for example ロンドンに日本のレストランがあります.

To my ear 私が今日本にいます suggests there are more people and we want to indicate that we are the one who's in Japan right now - since there is no context given and the sentence is supposed to be neutral, that's not the correct answer but I wasn't able to explain why we can't use が here if, in general, います requires が. Help?

Without any context, they both are correct. Generally speaking, は will refer to information that is already known, whereas が will introduce new information. が is also grammatically linked to the verb as a subject, while は as topic does not.

Thanks RandomQuotes

This trips most people up. Here's a pretty good article about wa/ga: http://nihonshock.com/2010/02/particles-...wa-and-ga/

Savii Wrote:This trips most people up. Here's a pretty good article about wa/ga: http://nihonshock.com/2010/02/particles-...wa-and-ga/

I like that article, it sums up bits and pieces I've learned over the years in one tidy bundle.

It also shows the increasingly popular name example which I think is almost identical to this situation.
In context the two answers would be correct like this (ignoring the fact that あなた and 私 would most likely be dropped entirely...),
(on a personal phone call)
あなたは今どこですか - where are you now?
私は今日本にいます - I'm in Japan now. ( meaning, Japan is where I am now.)


(on a conference call)
誰が日本にいますか Who's in Japan?
私が今日本にいます I'm in Japan now. ( meaning, The one in Japan right now is me.)

The first response is correct in 'neutral' context because it's correct in almost any case. The second response is only correct in a few cases, mostly (entirely?) when the listener knows that someone is or might be in Japan and you want to assert that that someone is you.