What is the differences in nuances between とうとう and やっと?
is it true that やっと has a more "happy" feeling attached to it?
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Sign up to join this communityやっと means that something realizes after spending/waiting a long time for it or making a lot of effort for it. Therefore it is understandable that やっと has a happy feeling attached to it. If I understand English correctly, “at last” has a similar meaning.
とうとう means that something happens as a final outcome, and what happens can be either a good thing or a bad thing.
For example, suppose that some company has been financially in a trouble for a while. Today you heard that the company went bankrupt.
あの会社、とうとう倒産したそうだよ。 (あのかいしゃ、とうとうとうさんしたそうだよ。) I heard the company finally went bankrupt.
is a usual sentence. If you say
あの会社、やっと倒産したそうだよ。 (あのかいしゃ、やっととうさんしたそうだよ。) I heard the company went bankrupt at last.
that means that you were waiting for the bankruptcy of the company.
やっと校長の話が終わった。でも、まだこれから教頭の挨拶があるんだ。
Toto is crucially different from yatto in that the former often indicates a negative situation that came about spontaneously, but the latter indicates a positive situation that has been realized with the greatest efforts. {From intermediate dictionary on Japanese grammar pg 530}
so it seems you are right that yatto is for happier occasions.