Translation:Good morning, Grandfather.
Ok so I think I just figured this out (regarding aunt, uncle and grandparents) and want to share it, because it caused A LOT of confusion for me:
伯父(さん) = おじ(さん) = oji(san) = uncle (anyone older than one's parent)
伯母(さん) = おば(さん) = oba(san) = aunt (anyone older than one's parent)
お祖父さん = おじいさん = ojīsan = grandfather
お祖母さん = おばあさん = obāsan = grandmother
祖父 = そふ = sofu = grandfather (informal)
祖母 = さぼ = sabo = grandmother (informal)
お爺さん = おじいさん = ojīsan = a very old man
お婆さん = おばあさん = obāsan = a very old lady
As you see, お祖父さん + お爺さん and お祖母さん + お婆さん are pronounced the same. Therefore, by writing them with katakana, you leave it open whether it's your ancestor or just an equally old person.
Thanks for consolidating everything into a post!
However, you have a typo for "sobo" for 祖母, you might want to edit it, as well as the other posts which you copied and paste
And I want to point out, those 4 aren't pronounced exactly the same. I feel おじさん is pronounced much faster than おじいさん, while some say that the tone for さん varies between those 2. Applies to おばさん too
This is the only excersice were there's only one answer. Things I tried:
「おはよう、お祖父さん」 「おはよう、お爺さん」 「お早う、おじいさん」 「お早う、お祖父さん」 「お早う、お爺さん」
They all mean the same and are pronounced the same for what I investigated. Is there a particular reason why kanjis are not accepted (Other than おじいさん being the usual way to write it, I mean)?
Wow I was about to experiment half of those myself but luckily I checked the forum first hahaha
Adding onto the question, is お祖父さん and お爺さん used in formal and informal writings respectively?
I'm guessing based using the Chinese characters as 祖父 and 爷爷 is used for formal and informal context
We use お母さん and お父さん to address directly within own family because they are older than us.
In Japanese culture, there are hierarchical relationships and it can be different from formality. If we talk to the boss or a schoolmate in a higher grade, it has to be upward and is most likely to be formal (です, ます, keigo), unless we are a very close friend with that person. However within a family, we don't use です or ます unless we are in a hostile relationship.
「叔父さん or 伯父さん」・おじさん ojisan would be "uncle" with a short い sound
「 お祖父さん or お爺さん」・ おじいさん ojiisan would be "grandfather" with a long い sound
The way of pronouncing them each is different enough to distinguish in conversation, but may be difficult for learners to pick up on when given both words separately.
It's a bit easier to tell the difference if you hear them back-to-back:
KeithWong9 also discusses how to differentiate them in the top comment https://forum.duolingo.com/comment/23526116$comment_id=38852904