"Good morning, Grandfather."
No it won't, because you're addressing the grandfather here directly and you always show respect to older people in Japanese by using the polite version. So it's お祖父さん（おじいさん）and not the humble form 祖父（そふ）. The latter you'd use when talking about your grandfather with someone else.
As I understand it, you would use the humble version if you were speaking to someone outside of your family. For example, after a son was born, a father might have tell a friend "我々は祖父の名前を息子に名付けた", meaning "We named our son after my grandfather"
No, there's a difference!
casual (informal) < polite < honorific or humble form
Casual (informal) language is what you use with friends;
Polite speech you use with strangers, work colleagues, etc.
and honorifics you use to adress people of a higher social standing than you, like your boss, senior citizens, teachers, etc., but it's also how shop employees would speak to costumers (since the costumer is king or literally god in Japan 「お客様は神様です」).
The humble form is used for when you refer to yourself (and your in-group/family/etc.), to express modesty as well as show respect to the person you're speaking to and lift them higher so to speak.
Hope this clears things up.
Sofu, the humble form, is used, when you talk about your own grandfather with someone else to show respect to the listener by not putting your family members over theirs.
Ojiisan, the polite form, is used, when you adress your own grandfather directly to show respect to older people or when you want to refer to someone else's, to show respect to their family members.
Found this from a quick google search:
Chan (ちゃん) is a form of san used to refer to children and female family members, close friends and lovers. The change from san to chan is a kind of "baby talk" in Japanese where "sh" sounds are turned into "ch" sounds, such as chitchai for chiisai, "small".
Makes sense, seems fairly accurate?
In sentence you address your grandfather directly and are not talking about him with someone else.
See my comment above where I already explained this or take this link:
Also another one of my explanations about the different politeness levels in this same thread:
It should. Try reporting it as possible answer with the flag button. Whether or not you'd use the more polite version with ございます really depends on your relationship. Plus I'd argue putting the addressee first in the Japanese sentence is more natural anyway, that's how I always answer despite the model answer putting it at the back, when you look at the top of this discussion.
So, the word bank included both "祖父" and "おじいさん". Both of these mean "grandfather". My understanding is that "祖父" is more lkely to be used to refer to your own grandfather (ie: it is the humble form), while "おじさん" is more polite (formal). The app, however, only accepted "おじさん." Does any one know if this is because the application is looking for the hiragana vs the kanji, or if it is because it understands it is more appropriate to use the more respectful""おじさん" when speaking to your grandfather?
so a few practice sentences ago i used the formal "ojiisan and obaasan" when it was clearly referring to my grandparents yet i was marked wrong... must use casual it seems since they're my family no need to be formal.
but now i that i used the "sofu" since its implied that i'm talking to my grandpa it also marked wrong... wtheck duolingo?!!
see my explanation in the comment above or use this link https://forum.duolingo.com/comment/23662840?comment_id=38572400
Use the informal for when you're speaking about someone in your own family to someone (who is not the person you're speaking of). Use the formal for when you're either speaking about someone else's family member or you're speaking to that person directly
e.g. お祖父さんは何歳ですか。 "How old are you, grandpa?" Or "How old is your grandfather?"
祖父は80歳です "(my) grandfather is 80 years old"
Your explanation is correct, however it's not informal vs formal, but humble vs honorific form (which are both formal) .
See my explanation here: https://forum.duolingo.com/comment/23662840?comment_id=40795386
Well, there are no spaces in Japanese (super rarely between last and first name, if they're else too hard to read) and Kana and Kanji already logically divide words well enough to be read without problem. Plus when typing, Japanese punctuation marks occupy their own squares and make enough room this way automatically.
Edit: Forgot to mention, I was trying to say that the space caused it to be rejected and not the missing comma (because Duolingo ignores punctuation most of the time).