which sentence is better?

第一句的用法 (用comma), 一般係前後句的意思相反, 而作對比。

例:
I thought the questions were very easy, but then again I'm a genius.
雖然問題好易, 不過o禁, 我都好聰明!!

I found their conversation very dull, but then I have different tastes.
我發覺佢地的對話好悶, 不過o禁, 我都覺得有另一番味道o既!!

but then = but on the other hand 但雖然係o禁 (在另一方面), ...

但係, 第一句的用法 (用comma), 前後句並無相反的意思 (在巴黎住了3年, 法文講得好係好正常), 所以應該係第二句的用法較好。因為, 當你要在句子中忽然加插另一個想法時, 就可以用dash。

例:
I think you would look fine wearing either the silk blouse -- the one with the blue pattern -- or the angora sweater. (Abrupt interruption)

We pored over exotic, mouth-watering menus from Nemo Catering, Menu du Jour, Taste Temptations, and three other reputable caterers -- and rejected them all. (Dashes also mark sharp turns in thought).


圖片參考：http://tw.yimg.com/i/tw/ugc/rte/smiley_1.gif




2008-08-08 15:16:46 補充：
我意思係話

因為後句 "but then she did live in Paris for 3 years" 係說話者忽然醒起要加插o既補充資料 (補充佢前句: 點解她的法文講得這麼好), 所以用dash 會較適合。

thanks for jeanniehy, 克成, wonghangtat2002, and erictang007

you did give me some very good improved sentences

however, the main question is ---> actually, i don't know i should use "," or "--"

Both don't make sense,

She speaks good French BECAUSE she had lived in Paris for three( when you write, you should use words, not numbers) years.

two is wrong~
she speaks very good French, but then she did live in Paris for 3 years
you should add could before speaks
change but then to because or as
she could speak French well because she have lived in Paris for 3 years.