Καθαρεύουσα, as an official language of the Hellenic Republic up to 1981, was more less very closed to the Attic form, particular in its written form.
the sounds of H & Y for a non trained ear are close, and hard to distinguish,
but when one says ΝΥΧΤΑ - ΝΥ
Ξ, or Η
ΛΙΟΣ then it shows the difference.
The lower case letters indicate the difference.
in the υ
the sound moves down, stays down and then comes up, while in η
the sound moves down and up immediately, then while up moves down again.
the way one writes the letters indicates the way that the sound moves.
BACK to Καθαρεύουσα.
If one looks in google books, books published up to the period before the second world war, it is more close to classical Greek,
From the WWII and after it moves slowly towards the more relaxed forms of demotic greek, particularly in oral forms first and written later.
Before the first world war things were very closed to classical, see the journal ΑΘΗΝΑ as well as in printings in Smyrna, Alexandria Egypt, Constantinupole, Jerusalem, Odessa, Venice, Vienna etc. The level of education of Greeks outside of the Hellenic state was much higher than within the Hellenic state. This reflects the usage of the language as well.