This is one of those questions that seems relatively simple, when should you use än and when should you use ännu? Or are they just the same word? Quick answer, when än acts as an adverb the two words can generally be considered synonyms and you can usually switch between the two, so feel free to use ännu. Or use än if you prefer. And no.

If that’s all you need, you’re good to go. Feel free to continue reading about the Swedish language here on the site. If you want just a little more detail, keep reading.

The word än can be an adverb (Klockan är inte fem än), a conjunction (Jag äter mer godis än jag borde), or a preposition (Hon är kortare än dig). There is sometimes debate about whether or not the subject form or object form should follow än, which is the main difference between än being used as a conjunction or a preposition. Språkrådet and Svenska Akademiens ordlista both try to put that to rest and state that än can be used as an adverb, a conjunction, or a preposition.

It’s clearly a versatile word, but it gets confusing when we introduce ännu. In the above sentences, we can only replace än with ännu in one of them:
Klockan är inte fem än (It’s not five yet)
Klockan är inte fem ännu (It’s not five yet)

Or a couple of more examples:
Det lever kvar än idag (It still lives on today)
Det lever kvar ännu idag (It still lives on today)

Tack än en gång för hjälpen (Thanks again for the help)
Tack ännu en gång för hjälpen (Thanks again for the help)

Or this example from Sveriges Radio
“…kongressen kommer att bli än mer polariserad…” (…Congress will become even more polarized…)
…kongressen kommer att bli ännu mer polariserad… (…Congress will become even more polarized…)

So we see that in all of the examples above, än is being used as an adverb and thus can be switched out for ännu. Keep in mind though that all of those examples are perfectly ok regardless of your choice of än or ännu. If you prefer to use ännu, that works. If you prefer to use än, that works too. When än acts as an adverb, än is synonymous with ännu. If you’re planning on using än as a conjunction or preposition though, DO NOT use ännu.