Still, again and now and then (wciąż, ciągle, od czasu do czasu)

Still going on

still (not yet ended)

wciąż — still — says that a state has not ended. It is the plain word; nadal (which you know) is its slightly more formal twin.

constantly, continually

ciągle — constantly, all the time — stresses repetition, often with a note of irritation.

all the time, the whole time

cały czas — all the time, the whole time — the neutral version.

Again

again (short form of znowu)

znów is the short form of znowu — again. Both are correct and equally common; znów is a little more frequent in writing.

Sometimes — and by any chance

sometimes (= czasami); by any chance (in questions)

czasem — sometimes, the same word as czasami. In a question it also means by any chance: Czy masz czasem ochotę na kawę?

from time to time

od czasu do czasu — from time to time: literally "from time to time", with od and do both taking the Genitive (czasu).

Forever

forever, for good

na zawsze — forever, for good: na + zawsze.