The meaning of ‘minna’ (みんな)
‘Minnna’ is written 皆 in a Chinese character, which is also pronounced ‘mina’. Then is there any difference in meaning between the two?
In fact, both mean the same! They mean ‘everybody’ or ‘everything’.
However, there is NO expression like ‘minnasan’ in Japanese; you have to say ‘minasan’ [mina ‘everybody’ + -san (a polite suffix)].
‘Mina’ is the traditionally correct pronunciation, and ‘minna’ is a more recent expression and more informal word used in conversation. If you watch historical Japanese dramas called ‘jidaigeki’ (時代劇), you will hear characters in authority always speak of ‘mina’ or ‘mina no mono’ (みなの者 [a rather condescending expression, which means ‘everybody’.]), but not ‘minna’.
Compared to ‘mina’, ‘minna’ is a more informal everyday expression and a little more emphatic than ‘mina’. [See ‘minna’ in Meikyo Japanese Dictinary. 2002]
How and when are ‘minna’ and ‘mina’ used?
Children, for example, use ‘minna’, but hardly ever ‘mina’. That is because their teachers almost always use ‘minna’, and children imitate that expression. (You have to remember, though, that even teachers say ‘minasan’ with ‘-san’, never ‘minnasan’!)
Besides teachers, ordinary adults also address children as ‘minna’ (and ‘minasan’).
However, because adults also use ‘minna’ in their everyday conversations, ‘minna’ is not a childish expression at all.
‘Minna’ is a colloquial expression used in conversation by most Japanese.