Fill Vowels

Fill Vowels are also called Mobile Vowels and Fugitive Vowels. They come and go inside Russian words as they decline or conjugate, so that отец suddenly loses its E and becomes отца, while земля as suddenly gains an E to become земель.

Why? What's going on here?

 It's all explainable - if a bit historical and arcane. And it starts with "The Fall of Jers". (And no, "Jers" isn't a city, which was my first thought on hearing the phrase!)

Until roughly a thousand years ago, the Slavic ancestor of Russian had two more vowels – written with the letters ъ and ь, ‘uh’ and ‘ih’ respectively. These very short vowels, which linguists call ‘jers’, eventually suffered the same fate as the final ‘silent e’ in English words like, well, ‘like’ – they stopped being pronounced. Some of them turned into other vowels (mostly O and E) but some simply dropped out of the word entirely. And the ones that remained ceased being vowels entirely and took up their current role as diacritics.

This process - losing these short vowels - meant that Common Slavic, which, up until then, had ended every syllable with a vowel, now had syllables ending in consonants. It also meant that the CC non-syllabic roots came into being. And that is where fill vowels come into play.

For example, take a word like сънъ. Remembering that the ъ was a vowel, this word would have been pronounced ‘suhnuh’. When jers were lost from a word, if there were more than one, the last one, bearing the least stress, was lost; the preceding one was converted to another vowel. So, сънъ became сoн. But in other cases, there was a ‘real’ vowel at the end – genitive сънa – and so it was the other that was lost: сна.

Similarly дьнь became день* and дьня became дня. While in feminine and neuter nouns – with their strong vowel endings – it’s the genitive plural that changes. For instance, сестъра becomes сестра, while сестър became сестер.

The most common fill vowel is O: Examples: ОКНО – ОКОН; ШУТКА – ШУТОК; ОГОНЬ – ОГНЯ Sometimes the O is a Ё: Examples: ЛЁД – ЛДА; СЕСТРА - СЕСТ ЁР; ПА ЁК – ПАЙКА

The next most common fill vowel is a genuine E: Examples: ЧЕЙ – ЧЬЯ; ДЕНЬ – ДНЯ; ЗЕМЛЯ – ЗЕМЕЛЬ; БЕЙ – БЬЮ

И** is more rarely seen.

In adjectives and nouns, it is used only next to a jot. Examples: ТРЕТИЙ – ТРЕТЬЯ; СОБАЧИЙ – СОБАЧЯ; ЯЙЦА – ЯИЦ

In verbs, И is found in imperfectives formed from prefixed perfectives: Еxamples: СОБРАТЬ – СОБИРАТЬ; УМРЕТЬ – УМИРАТЬ; ПОМНИТЬ – ПОМИНАТЬ.

**Note that in six verb roots the fill vowel is Ы:
З/В: ПРИЗВАТЬ – ПРИЗЫВАТЬ
М/К: ЗАМКНУТЬ – ЗАМЫКАТЬ
Р/В: ПОРВАТЬ – ПОРЫВАТЬ
С/Л: ПОСЛАТЬ – ПОСЫЛАТЬ
С/П: ЗАСПАТЬ – ЗАСЫПАТЬ
Т/К: ЗАТКНУТЬ – ЗАТЫКАТЬ

 * Remember, the Ь here is marking the final H as soft; it's not a vowel.

Verbs

In the "non-past" (that is, the present and future), Russian verbs inflect (change) for Person and Number. There are two main conjugation types, called Type I and Type II, which are distinguished by the vowel used in the endings. Endings are added to the stem, which is generally the infinitive less the ТЬ ending.

(Thus, as for nouns, when we say we are "adding an ending" what we are doing is actually changing one, or replacing one - in this case, replacing the infinitive ending with the present-tense one.)

The textbook uses "Type IB" for verbs whose stems undergo a mutation (change) making them hard to detect in the infinitive.

The basic endings are -У -ЕШЬ -ЕТ, -ЕМ -ЕТЕ -УТЬ for Type I, and -У- ИШЬ -ИТ, -ИМ -ИТЕ -АТЬ for Type II. The У and A may be written as Ю and Я if the preceding letter is a soft consonant or another vowel, and the ET will be ËT if stressed

Verbs we have encountered so far:

Infinitive	Я	ТЫ	ОН ОНА ОНО	МЫ	ВЫ	ОНИ
Type I (regular stems)
читать read	читаю	читаешь	читает	читаем	читаете	читают
знать know	знаю	знаешь	знает	знаем	знаете	знают
изучать study	изучаю	изучаешь	изучает	изучаем	изучаете	изучают
Type II
говорить talk	говорю	говоришь	говорит	говорите	говорим	говорят
курить smoke	курю	куришь	курит	курите	курим	курят
Type IB (irregular stems)
ехать go, ride	еду	едешь	едет	едем	едете	едут
жить live	живу	живëшь	живëт	живëм	живëте	живут
звать call	зову	зовëшь	зовëт	зовëм	зовëте	зовут

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Resources:
A web tool for conjugating verbs
From Master Russian, the 500 most common verbs fully conjugated: alphabetical list and list by how common they are
Russian Lessons.net's list of 36 common verbs in all their forms
Master Russian's lessons on verbs
Russian for Everyone's lessons on verbs
Andrey Zaliznyak's (rather complicated) Classification of Russian Verbs