- they understand what you mean.
- they don't want to be impolite or make you feel bad.
- they don't want to end up in the situation where they have to talk about Finnish grammar.
Say hyvää when you are eating or drinking something and you want to express that it tastes good. Hyvää is also used when the question word requires it. Notice that the translations are horrible on purpose.:)
- Mitä sulle kuuluu? - Hyvää. - What is hearing to you? - Good.
- Millaista ruokaa siellä oli? - Hyvää. - What kind of food there was? - Good.
Hyvä! is used when reacting to incidents and news. Unless you are eating or drinking something, this is usually the one you should use.
- Mä jätin mun poikaystävän. - Hyvä! - I dumped my boyfriend. - Good!
- Tämä korjataan huomenna. - Hyvä! - This will be fixed tomorrow. - Good!
(Confusingly, if you want to use any other adjective as a one word expression, they are in partitive. Mahtavaa! Hienoa! Kamalaa! - Excellent! Great! Horrible! )
- Miten sä voit? - Hyvin. - How are you feeling? - Well.
- Miten menee? - Hyvin. - How's it going? - Well.
- Kuinka työhaastattelussa kävi? - Hyvin. - How did the job interview go? - Well. (Käydä is to happen in this context.)
Hmm.. I have a feeling that I will be editing this post a lot. Feel free to leave questions and comments. :)