Acute Accent An acute accent means: the marked syllable is short and sharp, on a rising note or pitch.
On the last syllable of a word it is replaced by a grave unless followed by a punctuation mark or an enclitic (=word that leans on it).
Where do we find it?:
  on ultima (last syllable) bla-bla-bla-blá  Examples
  on penult (last syllable but one) bla-bla-blá-bla   Examples
   on antepenult (last syllable but two)
      but only if the last syllable is short
bla-blá-bla-bla   Examples


Circumflex         The stress put on the marked syllable is drawn out. Always pronounced LONG. In theory the voice goes up, then down again, leading to a singsong sort of intonation. Which is difficult to do unless a 4th century b.C. Athenian comes along and lets us imitate him. I stick with LONG.
But keep in mind that it most probably SHOULD be pitch going up, then down again. Which will explain why words ending in stressed -ων take an acute accent, unless they are genitive plural or otherwise the result of a contraction when they DO take a circumflex.
As we'll see later on. Just keep an open mind about the seeming vagaries of pronunciation and spelling.

Where do we find it?

  on ultima  bla-bla-bla-blâ Examples
  on penult (only if última is short)          bla-bla-blâ-bla Examples
  NEVER on antepenult