i don't understand "hadn't learned better"here, what does it mean？
Ma Baxter sat at the table wait-ing for them, helping their plates. Her bulky frame filled the end of the long narrow table. Jody and his father sat down on either side of her. It seemed natural to both of them that she should preside.
"You-all hongry tonight?" she asked.
"I kin hold a barrel o' meat and a bushel o' biscuit," Jody said.
"That's what you say. Your eyes is bigger'n your belly."
"I'd about say the same," Penny said, "if I hadn't learned better. Goin' to Grahamsville allus do make me hongry."
"You git a snort o' 'shine there, is the reason," she said.
"A mighty small one today. Jim Turnbuckle treated."
"Then you shore didn't git enough to hurt you."2. what does "allus"here mean?3.what does "you git a snort o'' shine there"mean？4.in“"Then you shore didn't git enough to hurt you." ， does "shore" mean "sure?" or "must be?"

Question

i don't understand "hadn't learned better"here, what does it mean？
Ma Baxter sat at the table wait-ing for them, helping their plates. Her bulky frame filled the end of the long narrow table. Jody and his father sat down on either side of her. It seemed natural to both of them that she should preside. 
"You-all hongry tonight?" she asked. 
"I kin hold a barrel o' meat and a bushel o' biscuit," Jody said. 
"That's what you say. Your eyes is bigger'n your belly." 
"I'd about say the same," Penny said, "if I hadn't learned better. Goin' to Grahamsville allus do make me hongry." 
"You git a snort o' 'shine there, is the reason," she said. 
"A mighty small one today. Jim Turnbuckle treated." 
"Then you shore didn't git enough to hurt you."2. what does "allus"here mean?3.what does "you git a snort o'' shine there"mean？4.in“"Then you shore didn't git enough to hurt you." ， does "shore" mean "sure?" or "must be?"

Answer

The dialogue here is a phonetic transcription meant to represent a certain regional dialect of English. Please do not use this as a reference for how to write standard English, or how to spell these words.

The writer is writing in order for the spellings to match how the people in this region pronounce these words. The truth is, even if they pronounce the word as "hongry", speakers of this dialect still spell the word as "hungry".

Reading things like this will be very difficult if you have not lived around these specific stereotypes, and can lead to mistakes in your writing if you are not careful.

With that said, I will answer your questions. 
1). I think that "I'd say about the same,... If I hadn't learned better" Here means "I would think that I could eat that much too if I hadn't already learned my lesson." The character is saying that they learned that they can not consume the amount of food that Jody is suggesting.

2). I had to look this one up, but "allus' is apparently the word "always" 
In standard English, the passage would read:  "Going to Grahamsville always does make me hungry"

3). I'm not 100% certain, but I believe "shine" is referring to "moonshine" here. If it is, it is likely that "a snort o' shine" means "a bit of moonshine"

4). "shore" here is the word "sure". saying "you sure didn't get enough to hurt you" would mean "you did not consume enough to hurt you" in a literal translation, but I feel that I may be missing an idiom here with the phrase "enough to hurt you"

Like I said, texts such as these will be very difficult to understand. Even as a native speaker in English (and one who enjoys reading old texts and language), I have trouble understanding precisely what is said in this passage because I don't speak or hear this dialect of English in my daily life.

Hope this helps :)