Hellos and goodbyes
Moi, terve, moro, hei hei. Which one you use says where you are, how old the other person is, and how well you know them.
Finland has more greetings than it has any obvious need for, and which one you hear tells you something. Moi dominates the south and centre, terve runs east and around Oulu, moro belongs to Tampere. Doubling any of them turns hello into goodbye, and nobody will ever explain this to you.
Age does as much work as geography. A shop assistant in Helsinki will say moi to someone young, hei to someone middle-aged and päivää to a pensioner, three greetings in one shift, each chosen in the second before speaking. These pages say which is which and what happens when you pick wrong.
Read the greeting you are given, then match it
The safest strategy in this whole category is to answer in kind. Whatever came at you is, by definition, the greeting that person considers appropriate for this room and this relationship, and returning it is both correct and a small signal that you were paying attention.
The failure mode is not rudeness, it is temperature. Moi to a consultant you have just met is not offensive, but hyvää päivää would have read as respect. The opposite error is more common among learners and funnier: hyvää päivää to a friend, which is stiff enough to get a laugh.
Worth knowing too that Finnish has no working formal "you". Teitittely exists and is almost entirely ceremonial, so the politeness axis here runs through the greeting and through word choice, not through the pronoun. Advice written for a Spanish or German learner does not transfer.
Common questions
What is the difference between moi, hei and terve?
Meaning, almost none. Distribution, a lot. Hei is the most common nationally, moi dominates southern and central Finland and is the Helsinki default, terve runs east and around Oulu and skews male. All three double up to mean goodbye.
Why do Finns say the greeting twice when leaving?
Doubling turns a greeting into a farewell: moi opens, moi moi closes, and hei works the same way. It is completely regular and completely unexplained in most courses, which is why the goodbye keeps catching people out.
Do I need to greet strangers in Finland?
In a shop, a lift or a small-town street, yes - a short moi or päivää is normal and its absence is noticed. On a Helsinki tram, no. The rule is roughly: greet when you are the only two people in the situation.
Is hyvää päivää too formal for everyday use?
For friends, yes, noticeably. For a first meeting in an office, a doctor, or anyone considerably older, it is exactly right. It is the one greeting that is never wrong for being too casual.
One word.
Every morning.
One real Finnish word every morning - what it means, what the book form is, and when Finns actually say it.