[#DuolingoForumGems originally posted on 2020-09-24 on the Duolingo Hungarian for English speakers forum by peter.kristof.hu ] 

Connecting vowels

In Hungarian, it is often the case that a vowel has been placed between the word and the suffix. This can be a connecting vowel (e.g. asztal-o-k), however, sometimes linguists consider it to be part of the suffix. Some connecting vowel harmonizes as is known in vowel harmony (asztalok, székek, tökök). Leading vowels of suffixes that do not alternate we do not call connecting vowels (e.g. -ig, -ért). Connecting vowels are usually short mid vowels (o, e, ö, sometimes a). The quality of the connecting vowel depends on both the stem and the suffix.

The first question is whether a connecting vowel is required between the particular word and suffix, and if so, why. It is obvious to assume that the phonological well-formedness justifies the appearance of the connecting vowel. Indeed, the connecting vowel has a role in facilitating pronunciation. There is no doubt that the Hungarian language cannot tolerate some consonant connections, so a vowel is inserted. For examples, the "cst" and "tm" linkage are undesirable: "ácsot" (carpenter, accusative), "rétem" (my meadow). Many vowel linkages, however, are not strange to the Hungarian pronunciation, yet connecting vowel required. Examples:
hal-ak -- halk, tan-ok -- bank, kér-ed -- térd, etc.

The need to use connecting vowels is primarily for historical, etymological reasons. It is not possible to predict whether a suffix will get a connecting vowel or not, therefore, language learners need to memorize these forms. (There is also a case where the word has two forms, possibly with some difference of meaning, e.g.: szók - szavak (words)).

1. Suffix without connecting vowel

• Connecting vowels are not usually used after the stems ending in a vowel. (kapu-k (gates) (exception: férfi-a-k (men)).
• A suffix beginning with a high vowel (u, ü, i) is not considered a connecting vowel but as part of the suffix (ház-unk (our house)).
• The leading vowel of a non-alternating suffix belongs to the suffix (e.g. -ig, -ért).

2. Suffix with connecting vowel

Connecting vowels can be: o/e/ö. They behave according to the rules of vowel harmony as the suffix -on/-en/-ön (except for the next paragraph). For suffixes, in a nutshell, the rule is that front vowel words take front suffixes, back vowel words take back suffixes, and mixed vowel words usually take back suffixes
(in details see:  https://magyarbagoly.blogspot.com/2022/02/vowel-harmony.html )
Note, it is not the stem, but the whole word with the ending that determines the vowel harmony. Compare connecting vowels with that three-form suffix:

ház-on -- asztal-o-k
ember-en -- szék-e-k
tűz-ön -- tök-ö-k

3. Opening stem

First, consider the phonological classification of Hungarian vowels:

Front	(Front) Rounded	Back
i, í	ü, ű	u, ú	Close/High
é	ö, ő	o, ó	Mid
e		a, á	Open/Low

Open-close: Position of the mandible. Low-high: Vertical position of the tongue towards the palatal area. ("E" is a particular case, sometimes considered as a mid vowel.)

Some stems, these are the opening stems, change the connecting vowels and make them open: o changes to a and ö changes to e. E.g.:

asztalok -- házak
fürdök -- tüzek

There is no rule as to which stem is an opening stem or not, but there are still some guidelines.

• There are a large number of opening stems between nouns and pronouns.
• Adjectives are usually all opening stems. Many adjectives fluctuate: undok-ok/-ak.
• There is no opening stem among the verbs (vár-ok, tűr-öd).
• There is always a connecting vowel before the plural ending, while only occasionally before the accusative ending. The plural ending always creates an opening stem.
• The class of opening stems is closed, i.e. foreign words that are newly added to the language can only be suffixed with the mid vowels (o, e,ö).

I know I just touched on the topic given its complexity. Instead of grammatical rules, here are some examples for nouns and adjectives:

	-k	-t	-on	-ban
ház	ház-a-k	ház-a-t	ház-on	ház-ban
ember	ember-e-k	ember-t	ember-en	ember-ben
emberek	-	emberek-e-t	emberek-en	emberek-ben
barna	barn-á-k	barn-á-t	barn-án	barná-ban
kék	kék-e-k	kék-e-t	kék-en	kék-ben

Sources:
https://mersz.hu/hivatkozas/m26smny2_s2.4.1#m26smny2_s2.4.1
https://m.nyest.hu/hirek/a-nyitotovekrol