到頭(とうとう)Finally, After all

とうとう is an adverb that is used to express that something 'finally' happens. In Japanese, there are a few ways to do this, but とうとう is one of the more common. Coming from the kanji 到頭(とうとう), this structure literally means 到 (to arrive at), 頭 (the head of/crux of) something. Due to this, 'finally', and 'after all' are the two most common translations of とうとう. Like many other adverbs in Japanese, とうとう is often used at the beginning of a phrase/sentence.

While とうとう is written in hiragana the majority of the time, knowing the kanji will help when trying to remember the expression's nuance. That nuance is that something has 'ended', usually after some sort of long journey/effort.

This adverb may be used to express either positive or negative outcomes, as it focuses more on the time required to achieve the result, rather than the result being good or bad.

Caution

とうとう is not used in situations where you expect something to happen/start, but there has not actually been a period of time in which some sort of effort was made to achieve a result.

A good example of this is the changing of seasons. You might want to say 'finally, it's summer!', but this is not something that anyone has done anything to achieve. Rather, it came naturally. In these cases, いよいよ will be used, and has the nuance of 'finally', or 'at last' (in relation to something starting/happening).