Translation:Welcome back, Dad.
"dadio" is kind of rare; people in america only say that in a joking manner, saying "dadio" as a non-existent word, similar to "correctamundo" adding "amundo" to it to sound 'cool'. adding "io" (--イオ）was considered cool around the 70's, it's slightly complicated to explain but people don't say that anymore.
now "daddio" only refers to 'old slang' that USED TO be cool. it isn't a real word.
again, "dadio" doesn't even mean dad, it's just a word to call another male like "dude"
Edit: okay, seriously... i got downvoted 6 times for this?
what he said was a joke, sure, but there are many people here who don't speak native English and they need to be informed that "dadio" is a word that no one would ever use... i was also explaining on the off-chance that he was legitimately confused and wasn't solely making a joke... but apparently jokes > learning here on duolingo...
When you're talking to other people, ABOUT your own dad / 父, it's pronounced ちち.
("My 父 did this the other day...")
When you're talking TO your dad, it's either お父さん (the 'お' in front makes it more formal) or 父さん, which are pronounced おとうさん and とうさん respectively.
("Hey, (お)父さん, can I buy this?")
I hope this made sense!
In Japanese, the command form is used to issue commands to others. However, it is quite rude so you should use it with caution (if at all).
Command form conjugation rules
For ru-verbs: Replace the last る with ろ
Example: 食べる + ろ = 食べろ
For u-verbs: Replace the last u-vowel sound with the e-vowel equivalent
Example: 買う + え = 買え
する → しろ
くる → こい
くれる → くれ (exception for this conjugation only, not an exception verb)
Imperative means you're making a command. I.e. in English, saying, "Go wash the dishes," is imperative, and uses an implied subject, "you." I'm not sure how this translates to Japanese, though, since so many things are implied from context already. I imagine, though, that it also means it's a command.
"daddy" is very weird for someone to say. ONLY kids under the age of like 5 and a 'sexual' meaning with a woman->sexual partner
the word "daddy" is kind of complicated. It doesn't actually mean 父さん, and if it did, it's a VERY little kid saying it.
my guess is that this actually originates from incestual fantasies, later becoming a common word (not usually said in public)
I thought お父さん is to refer to someone's father. So is he/she welcoming someone else father? Or is this a more polite way than おかえりなさい、父？
Not quite, お父さん is used about the fathers of other people as well. It is only when you talk about your own father that you use ちち. (You might think of it like this: you use お父さん to show respect to your own father when talking to him, and to other people when talking about their fathers. You don't need to show respect to yourself when talking about your own father, so then you use ちち.)
～なさい is the polite imperative verb ending. "Please do~"
Broken down further it itself is the imperative form of the honorific verb なさる "Do"
You'll also see this verb in other set expressions such as ごめんなさい "I'm sorry" (lit. 'please do a dismissal') and おやすみなさい "good night" (lit. 'please do a rest')
おかえり is an honorific form and stem/connective form of the verb 帰る・かえる "to return"
おかえりなさい is more literally saying "Please return"
Note that this expression is most likely an abbreviation of よくお帰りなさいました more literally "you have returned well" (like saying 'i'm glad you have returned')
Over time the よく "good, well" adverb and ～ました polite past tense ending have dropped to the expression we now use today.