いよいよ

いよいよ says that a moment you have been waiting for or preparing for is here, or is about to be: "at last", "the time has come". It is an adverb and goes at the start of the sentence or just before the predicate.

The event is one you knew was coming, good or bad, and the sentence carries the tension of its arrival. That is why いよいよ sits most often with a non-past verb, or with a noun and だ: the moment is now or just ahead.

いよいよ, やっと or ついに

All three translate as "finally". やっと is relief that something you struggled for has come. ついに reports the end of a long process, often a result that is already in. いよいよ looks at an expected event as it arrives, and does not suit an achievement.

• × いよいよ試験に合格した → やっと試験に合格した

"More and more"

With a change of state – きごう〜くなる, きごう〜になる, きごう〜てくる – いよいよ means the change keeps building, close to ますます. This sense belongs more to writing and news than to everyday talk.