About comparative and superlative of adj

I am learning about comparative and superlative of adjectives, but it is so complex. Maybe my textbook tells it too simple.

I wonder that:
(1) For adjectives of the first and second declension(-ος, -η/α, -ον),take δίκαιος for an example. Its comparative is δικαιό-τερος, δικαιο-τέρα, δικαιό-τερον, and I wonder where the ο in the “δικαιό” comes from. As a connective vowel, or considering that the suffix is -ς rather than -ος?

(2) I met a word ἄξιος, whose comparative is ἀξιώτερος, why ω rather than ο?(and superlative of φοβερός is φοβερώτατος)

(3) For adjectives of the third declension(in fact I haven’t learnt that), according to nominative, or genitive?

(maybe you think that they should be included in the textbook, but I use a Chinese one rather than English, and its quality is not so good.)

The -ο- in the words on -ος is a penult (see Wikipedia), in german this vowel is called a „Kennlaut“ („identification-sound“, „marker“ ???): This vowel is added to a root and marks the word as a noun.

So the construction is root - „Kennlaut“ (here: -ο-) -ENDING/SUFFIX.

See Smyth, Greek Grammar, page 86-87 (https://archive.org/details/greekgrammar0000smyt/page/86/mode/2up : borrow-book)


Smyth also explains the lengthening -ο- > -ω-.

thanks!

I have another question after reading the book: do you know the comparatives of κενός and στενός?

The book said that originally they are κενϝός and στενϝός so -ο shouldn’t be lengthened. but wikipedia lengthens them. I suspect that wiki is wrong(there are no pages about their comparatives), but I cannot find enough evidence.

Again I refer to a German grammar:

Schwyzer vol 1, page 534 f.(https://archive.org/details/griechischegramm0000schw_w8v9/page/534/mode/1up).

It seems -ωτερος and -οτερος are both acceptable in these cases.

Thanks!