How to say...?

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There's several ways of saying "hello" in Hungarian, as follows :

Hungarian (Hungary) Jó napot

Hungarian (Hungary) Jó napot kívánok

Hungarian (Hungary) [informal, to one] Szervusz

Hungarian (Hungary) [informal, to several] Szervusztok

Hungarian (Hungary) [informal, to one] Szia

Hungarian (Hungary) [informal, to several] Sziasztok

Hungarian (Hungary) [informal] Üdv

On the phone, it's "halló" (pronounced /hall-low/, where hall rhymes with call and awl, and low rhymes with flow). Theoretically, that's short for <hallom> "I hear it", and equally theoretically, it's the origin of the English use of "hello" to answer the phone. (Something about Edison having a Hungarian assistant.)

In person, it depends on the relationship of the greeter to the greetee(s).

Very familiar, almost vulgar: szevasz (/seh-vahss/), or plural szevasztok (/seh-vahss-tohk/, where oh is like "oh, I see").

Close friends and family: szia (/see-ya/), szervusz (/sehr-voos/ or sometimes /sehr-boos/, where eh is like in "eh, whatever").

Close friends and family, multiple people at once: sziasztok (/see-ahs-tohk/), szervusztok (/sehr-voos-tohk/).

Everyone else: depending on time of day, jó reggelt (good morning, /yoh reh-gehlt/), jó napot (good day, /yoh nah-poht/), jó estét (good evening, /yo ehsh-tayt/).

Everyone else, more formal: same as above, but adding kívánok "I wish you..." (/kee-vah-nokh/). So: jó reggelt kívánok, jó napot kívánok, etc.

Note on pronunciation: the stress is ALWAYS on the first syllable of each word. Make sure you get this right - even if you mangle the vowels, if you get the stress placed correctly, you'll sound more correct than if you get the vowels perfect but mangle the stress.

Most ordinarily one would say "szia" (pronounced "SEE-yoh") which is a corrupt form of the English "see you", but is actually used as "hello" when meeting someone. An older form is "szervusz" (SERR-voos) or "szevasz" (SEH-vuss).

The_lipsiot's answer is so correct and so complete that you don't need to wait for other answers.

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