Term for pairs of vowels such as А & Я, Е & Є, etc

Question

Words ending in either vowel are often treated similarly in a grammatical sense, so I assume that there would be a term for something like this.

Answer 1

As Follower said, the fact that Я, Є, Ї, Ю can start with the [j] sound is called “iotation”=“йотація”. (And the Я, Є, Ї, Ю themselves are called “iotified letters/vowels”=“йотовані літери/голосні”.)

There is no way to characterize the А→Я, Е→Є, І→Ї, У→Ю relation with a single word.

However, you can use “iotified counterpart”=“йотований відповідник” to describe, what is Я for А:

Йотованим відповідником для «А» є «Я», «О» ж не має йотованої літери-відповідника.
An iotofied counterpart for “А” is “Я”; but “О” has no correspondent iotified letter.

Accordingly, you can use “non-iotified counterpart”=“нейотований відповідник” to describe, what is А for Я. And “a/the ‘iotified-noniotified’ pair”=“пара «йотований-нейотований»” to describe the “А-Я”-like pairs.

Answer 2

I think, it is iotation (йотація). In this case, term for pairs of this vowels is iotified vowels (йотовані голосні).

About words ending and their grammatical sense: (Ukranian)declension [відміна].

Answer 3

Diphthong або "дифтонг" in Ukrainian, is a compound vowel. You can also have tripthongs or trigraphs as well with compound of three vowels. phonetics citation I don't know if a specific term for the Ukrainian й-compounds exists, but that would be the general term for a compound vowel.