This thread is about Japanese words, phrases, and ways of talking that you only noticed after coming to Japan.
i.e. things that you don't learn in grammar books etc.
Here are a few of my observations, 5 months living in Chiba, just outside Tokyo:
本当だ!
ほんとうだ!
Hontou da!
I would translate this as "Oh yeah!!", as in:
"Hey, there's a fly on your sleeve."
"Oh yeah"
I don't know if people say "oh yeah" the same way I do however...
But "hontou da!" basically seems to be used when you realise something you didn't know before after being told by someone else...
食べていいよ
たべていいよ
Tabete ii yo
it seems that in all the text books, "-te + mo ii" is used for permission.
for example,
"hon ni kaite mo ii desu ka?" - Can I write in the book?
"tabete mo ii yo" - You can eat it/Go ahead, eat it
However, actually it seems that the "mo" is always left out in real Japanese speech.
折れちゃった!
おれちゃった!
Orechatta!
The informal for "-shimaimashita" is "-shimatta", but in informal speech, this never seems to be used and "-chatta" is preferred.
That's all I can think of for now.
Please post more examples if you know any.