"Γεια σου αγόρι!"
I think I know this, though we should verify with Theo or another native speaker. the σας of Γεια σας means either that the phrase is plural, directed to more than one person like the vous in French - essentially, Hello you all (there is no equivalent in English). It is also used in the singular when speaking formally (again, like vous in French to one person). The σου is 'you' exclusively in the singular and is informal.
I moved to the southern US recently and have noticed that many of the blue collar local people here to actually use Y'all in the honorific sense that σας is used in Greek.
For example I often have the hostess at a restaurant tell me (even when dining alone) "I'll have a table ready for y'all in one second!"
While its certaonly not standardized in English as it is in Greek, it does seem thst at least reagonally people are using y'all that way.
From this native English (American) speaker's experience, "hello" usually stands on its own. Hello. You wouldn't add anything else to it.
Some exceptions: Someone in a subordinate position might say "hello, sir" or "hello, Mrs. Smith." And you might use it with other words if you weren't sure you were talking to the right person(s). "Hello, shipping department?"
If you wanted to use this sort of greeting on an informal basis with someone you know well, you would probably use "hey". "Hey, dude."
Hi works with names. "Hi, Anne." I suppose hello does too, but kind of in a friendly but formal tone. "Hello, John" reinforces the name you just heard. "Well, hello, stranger" meaning you haven't seen the person in a long time, but you know them well. You wouldn't say this to an actual stranger!. You could even get away with "hello, woman" to your wife if it was clear you were being a wise guy, but again you would never say this to a woman you didn't know well.
Your mileage may vary. But likely you won't need to add a generic descriptive noun after the word "hello." It's usually clear whom you're addressing.
Misconception, not any relation even it sounds almost the same :)
Γεια is "Υγεία"=Health. You know this word already https://en.wikipedia.org/wiki/Hygiene . The H before y not said in the word today, and for convenience, this word is "Γεια", informally, because "Υγεία" is different, formal or informal, it is used when it is necessarry to talk about somebody's health.
So the Greeks wish "To your health" when the meet somebody or leave somebody. It is so simple, I think.
When you drink a coffee or water in a cafe, you can say: Γεια μας or "Στην υγειά μας"=to our health. The same when you drink a drink, whatever it is.
P.S. About salutation:
Καλημέρα=before 12 am (about)
Καλησπέρα= after 12 am (about), afternoon, when you meet somebody
Καληνύχτα=evening, when you leave somebody at night
Γεια σου (inf.), Γεια σας (form.), when you meet or leave somebody, instead of the above.
There are more, if you like:
Ciao= Italian, and slung: Τσάγια= teas, just joking and cheerfully,
Χαίρετε=Formal, specially between olders,
Notice that the Ancient salutation was Χαίρε imperative of the verb Χαίρω (Ancient), now χαίρω disappeared, but still exists and used Χαίρομαι= I am glad, happy.
Χαιρετώ=Hello, a bit formal and stylistic.
All these can be used when you meet or leave somebody instead of Γεια.
Many youngsters mostly, use "hello" as it is in English, indifferently, between friends, but never formally.
You can accompany Γεια with the name of the person, nickname or κύριε (masc.) or κυρία= Mr., Μrs...., or the word "παιδιά" or "κορίτσια". It depends of the intimacy. The word "δεσποινίς"=Miss, not used, it is old-fashioned, but can be met.
Generally speaking it depends on the cirmustance, the disposition, the hour, the character etc. which form one can use. But it plays an important role how to salute somebody, as it happens in every language I think. A rude or an extremely polite or intimate salutation plays an important role in human relations.
Καλησπέρα is good evening and should be used after 6 PM. Due to a misunderstanding of the words "afternoon" and "evening" some Greeks have started to use it wrongly. Καλήν εσπέρα (oldfashionated) became καλησπερα. Εσπέρα an old word for βράδυ/ evening. Οι Εσπερίδες/ the Hesperides are the daughters of the goddess Night/ Νύχτα
Yeah, speaking of Hygiene : in French when we toast (choking glasses !) we say "A ta santé" (informal) or "à votre santé (formal or plural), or just "Santé", which means "Cheers" in english and means... health in French ! Like in Spanish with the "A tu salud" or the german "Zum Wohl" or the Irish "Slainte", all these terms refer to the health, I discovered that when I was travelling 25 years ago and I was delighted to...
It depends on the circumstances and the individual. It's different if you are casually meeting new friends or people you will do business with. Some people don't really care for formalities while others like to keep their distances. The right thing is to ask for permission: "Μπορούμε να μιλάμε στον ενικό;" You will usually be easily forgiven as a foreigner, whatever mistakes you make anyway. Most Greeks will be enthusiastic just to hear you speak our language.
We have to wait for a native Greek to answer. Meanwhile, maybe you could be more precise. Do you remember to greet him/ her after a while or do you want to leave him/ her after only 5-10 minutes? The first day of the week or of the month must have time for good wishes. For the great lucky day which is both, you must reserve at least half an hour
I enjoy waking up my Greek friends and neighbours, and everyone else with καλο μινα on the first of each month, but I only get the chance to do this on twelve days a year. The response is often as if they didn't realise it was a new month.
But I haven't heard of the first day of the week before. How would that go? καλη εβδομαδα perhaps?
It is pronounced, as is the γ in γυναίκα... For more audio files check forvo.com, e.g. https://forvo.com/search/%CE%B3%CE%B5%CE%B9%CE%B1/