N4
とうとう
Finally, After all
とうとう夏休みが終わる日が来た。
The day summer break ends has finally come.
Explanation
「とうとう」 means that after a long time or many ups and downs, things finally ended up a certain way.
Often for a regrettable result: 「愛車がとうとう壊れてしまった」 (My car finally broke down). 「とうとう雨が降ってきた」 (It's finally started to rain). It can also be used for a good thing you waited a long time for.
Mixed up with
「やっと」 is only for a good result you were hoping for: 「やっと合格した」. Not 「×やっと壊れた」.
「とうとう」 works for good or bad, and is often used for sad endings: 「可愛がっていた猫がとうとう死んだ」.
「ついに」 is more formal and is used when something big finally happens: 「ついに新薬を開発した」.