Diacritical marks.

Nice word, savour it, turn it around in your mouth and your mind.
A critic is a person who decides whether something is good, bad, or indifferent and διά is Greek for through, with the help of.
So diacritical marks, or diacritics, are distinguishing markers, little written signs that help us decide how to pronounce or stress words.

  1. long sounds (macron, a dash over the vowel)    ᾱ ῡ ῑ
  2. short sounds (breve, a little u shape over vowel) I don't find it on the Spionic chart, so I can't type it in Spionic. But I can in Unicode, though, again, it makes for complicated typing.
    This is what it looks like, in Unicode: ᾰ ῐ ῠ
    You'll see it in your book, declension of θάλαττᾰ page 34
  3. short stress: acute (small slash) ά  ú   or
    grave (small backslash) like ὸ  ὶ
  4. long stress (circumflex, the frown-like thingy somebody said very aptly)
    εῖ  ᾶ
  5. aspirated sound: looks like a little c over the vowel and sounds like an h (in hoi polloi, Hercules etc.)
    ὁ  αἱ 
  6. soft sound, absence of h sound. ἐ ἠ
    Not really necessary, but some nice people, when thinking up these diacritical marks must have felt sorry for poor, naked-looking initial vowels without any distinguishing features, so they gave them an inverted c (or comma), to show that they weren't aspirated.
  7. diaeresis ¨ over an ι or an υ , like this ϊ and ϋ to show that they are NOT part of a diphthong, but pure vowel sound. πρωϊ for instance, meaning "early" and actually written with two diacritical marks, either acute πρωΐ if last word or before punctuation, or grave πρωῒ if followed by any word other than enclitics.
  8. iota subscript. This little sign does NOT affect pronunciation or stress, it is just a reminder that once upon a time there was a real vowel i . It's gone with the wind, leaving the Cheshire cat's smile behind. And the associated vowel is always long. Obvious, since it is supposed to be a diphthong and diphthongs are always long. Like:
    ῥᾴθυμος ἐν τῷ οἴκῳ  idle in the house