a "wa" question I'm affraid, sorry. I have been told that わたし は かたかな は まだ べんきょう していません Would mean roughly, "I have not yet learnt katakana." I am interested by the double appearence of は in what I think is it's "wa" role. Is this what's going on and if so how come? よろしくおねがいします。
わたしは かたかなは まだ べんきょうしていません。（WATASHI WA KATAKANA WA MADA BENKYOUSHITEIMASEN) = I haven't studied katakana yet.
There are two WAs used in the above sentence. The first WA is a topic marker. Normally an object marker O is used before BENKYOUSURU (to study), however, WA can be used instead to specify or stress the object (katakana). So it sounds like the speaker is studying a different writing system now (e.g. hiragana), but he hasn't moved on to katanaka yet.
not specifically negative but rather Kinzokusei no Inu
As Kanako said, the reason to use the は in place of the が is to place more emphasis/stress by upgrading the secondary subject to a primary subject... it's not really related to it being negative in the example you gave.
For example: In the case someone asks you: "Whatcha doin' this weekend?" you could answer 今日予定がないが、明日ある。 - I don't have plans today but I do tomorrow. 今日は予定がないが、明日はある。 - (As for) today, I don't have plans, but (as for) tomorrow I do.
In this case you can see that the word with the は stress is not related to it being negative or positive but rather emphasizing something as the subject. In the above example the second sentence including the は emphasis is the more correct of the two.
...or if you want an example with the は marking the plans instead of a day, here's one: "Do you have any plans this weekend?" 用事とかはあるけど、予定というのはない。 - I do have errands and stuff but (as for) plans per se, not really.
hmmm....yes, I do follow you on that. The question reminded me of some guy's grammar logic I remember reading years ago, so I thought I'd throw that out there. I spent a few minutes unsuccessfully trying to isolate where exactly I got that from. Out of one of those Kodansha Power Japanese series books I believe, though I might be mistaken.
I've never personally had much patience to go deep on は and が but occasionally I muse about the linguistic theoretics that may apply. It seems to just become naturally intuitive the more one speaks Japanese.
Although this isn't from whatever source I was attempting to refer to before, it is relevant to what I was talking about, so I post a direct quote taken from another book I was reading through tonight. From a chapter entitled "Describing the Existance of Animate And Inanimate Things" (in other words, using いる or ある)........
"The particle は may replace the particle が in a negative sentence if the speaker wishes to suggest that although one person or thing (marked by は) may be absent, other similar people or things are or will be present."
Taken from "Japanese Sentence Patterns for Effective Communication" by Taeko Kamiya