My textbook teaches it as tui3 in both, and the only variation I was able to track down on dictionaries was ui3. Does anyone know about any other Hokkien dialect using tu3 or distinguishing two different forms of 對 (if used in other dialects) as that 台南 speaker?
It should be tui3 in both contexts. In the FROM context, it's interchangeable with ui3 and an3. From your post, my guess is that he was talking fairly fast and tui3 came out as tu3.
Another thing I found intriguing is that he pronounces 日本 as git8-pun2 instead of jit8-pun2 or lit8-pun2. I should have asked him how he pronounces 雖然...
This is common in and around 高雄 Takau / Kohiong: /j-/ merges to /g-/ before /-i-/. The /g-/ is often very weak. Another trait of that "accent" is that the mid-level tone is relatively high, almost as high as the high-level.
I'm under the impression that this is mostly a Takau-Pintong 屏東 thing. Then again, I haven't spent much time down there. Tailam 台南 vs Takau is analogous to Kyoto vs Osaka -- Takau is a bigger city, but Tailam is old and proud and does not seem to let Takau influence it.
The same feature is found in Ciangpo· 漳浦, in China. It seems likely that there's a connection, even though Ciangpo· migrants never formed a majority in any sizable part of TW... I've flirted with the /g-/ pronunciation and I can say that it is so easy that if U get used to it, U'll probably never go back to /j-/ (in either of its forms) or /l-/.
During a boring class I remembered a word with perhaps pretty much the same variation as 日本: 偌儕. Seems like 偌 can be joa7, goa7 or loa7 in Taiwanese.
Yes, joa7 is relatively rare in TW now, and goa7 is probably more common than loa7 islandwide even though the git8 pronunciation is limited to part of the South. This probably proves that sound change occurs unevenly...
Today, I got some help with pronunciation from a Taiwanese from 台南, and I wrote down his pronunciations 'cos although my textbook says pronunciation on CD/romanization is based on 台中, to me it sounds more like 台北. (nothing against 台北, just that his pronunciation resembles more that from another speaker I met, so since they're helping me out I decided to take their pronunciations as a model instead of that on my textbook

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It's not that simple, to put it simply.
First of all, in EACH of the counties of 台南, 台中, and 台北, U'll find almost the full of spectrum of Taiwanese Hoklo dialects -- in the speech of the old-timers. And when someone says they are from one of these places, they usually mean the county.
There's even variation w/i the cities. In Taipak, old-timers in 大稲埕 speak the "Taiwanese Amoy" accent, with Lâm'oaⁿ and Tâng'oaⁿ influences poss. depending on family bkgrd or neighborhood. A few stops up the Red Line in Sūlîm, in the old-timers speak with a Ciangciu accent in line with the North Coast and the foothills of Middle and Southern TW.
A "Mainstream Taiwanese" accent has taken the place of all the old dialects. This dialect started forming in the Japanese Era with the advent of radio and the railroad. As a rule, everybody under 50 or 60 either speaks Mainstream Taiwanese or something heavily influenced by it -- unless they live in a remote place or unless a more prestigious dialect was at play. The only prestige dialect was the Taiwanese Amoy dialect of Taipak, centered in 大稲埕 Toātiūtiâⁿ.
Everybody under 25 or 35 seems to speak Mainstream Taiwanese almost regardless of location -- if they speak it at all. DOOR is mûi in Gîlân to people over a certain age, but people in their 30s tend to say mn̂g.
Mainstream Taiwanese is basically "Ciangciu", and basically Southern. TW Hoklo in Taipak has been upheld for decades by migrants moving in from the South and the Middle. Taipak-bred kids Mandarize, then go work for companies in town, while economic centralization brings in more Hoklo-speaking migrants from the South, whose kids or grandkids will Mandarize ... and the cycle continues.
The Taiwanese Amoy dialect is more or less dead to the 30-and-below bracket, a frontline casualty of KMT Mando-chicanery. The Taiwanese Amoy accent "lived on" for a long time as the "standard" for Hoklopop, but I notice that in the last 10 yrs or so, the new artists seemm to be switching to singing in Mainstream TWnese.
Takau 高雄 and Taitiong 台中 were built by and under the Japanese. They never had the intricate dialect tapestry that Taipak and Tailam had. Migrants brought their dialects into town, and their kids learned to speak "mainstream". When Maryknoll says they present Hoklo in the Taitiong dialect, they're really saying they teach Mainstream Taiwanese.