I am a native speaker of a Slavic language (Kajkavian dialect of Croatian) and a soon-to-be professional linguist - I took a year's course on diachronic Slavic accentuation, which included reading some sizeable literature not easily acquired on the Internet (usually because it's not in English). I think I got a decent understanding of the topic, so that I can make a good summary of the literature I've read.
An Introductory warning:
The development of Slavic accentuation is not well understood - it's impossible to explain it solely by sound laws, due to it's intimate relation with morphology. The further one goes, things become less and less clear - when it comes to the development of (Balto-)Slavic accentuation from the Proto-Indo-European one, there's multiple theories per scientist...
Because of this, I won't be able to post a complete list of sound changes from PIE. to modern Slavic languages - instead, I will focus on explaining how the system works from a synchronic perspective, with some diachrony to tie the different languages together. Without further ado...
The basics of Slavic accentuation:
What are the basics of Slavic accentuation and how does it differ from more familiar systems of free, mobile accent?
Other Indo-European languages also possess a free, mobile accent, whether they inherited it from PIE. (Greek, Vedic Sanskrit) or developed it independently (the Romance languages, English). Due to their familiarity, the Romance languages will make a good comparison.
I speak of accent, because both stress and pitch accent behave similarly in the aforementioned languages.
- A free accent is one whose placement is not predictable from the phonological properties of the word. This means that two words whose phonological shape is the same can have a different placement of accent - in other words, placement of accent is phonemic (for this purpose, it doesn't matter whether the difference is lexical or grammatical).
- A mobile accent is one whose placement differs between different morphological forms of the same word. This can happen in both derivational and inflectional morphology - although English lacks mobility of the latter type (Therefore, mobile accent withing an inflectional paradigm is somewhat exotic to English speakers).
A comparison of Romance and Slavic accent systems:
So, English, Romance and Slavic all have a free, mobile accent system. What are the differences?
1. Scope of accentual mobility.
Due to historical reasons, in Romance, accentual mobility is limited - Latin accent was limited to the last three syllables of the word, which was further limited in Romance by syncopes and apocopes. Therefore, when accent moves within the paradigm, it will move one or two syllables:
eg. Spanish /espeˈθifiko/ "specific" vs. /espeθiˈfiko/ "I specify" vs. /espeθifiˈko/ "he/she/it specified
Again due to historical reasons, no such restriction exists in Slavic, and when the accent moves withing the paradigm, it can skip multiple syllables:
eg. archaic Čakavian /ˈnebo/ "sky" vs. /nebeˈsǎː/ "heavens", /ˈnagovoriːl/ "he persuaded" vs. /nagovoriːˈla/ "she persuaded"
Another difference is in the behavior of proclitics: in Romance, proclitics are never accented, while in Slavic, they can be accented (enclitics are never accented in modern Slavic). Furthermore, when the accent moves withing a paradigm, it can skip multiple proclitics:
eg. archaic Čakavian /ˈna ruːku/ "onto the hand" vs. /na ruːˈki/ "on the hand", /ˈi na ruːku/ "and onto the hand" vs. /i na ruːˈki/ "and on the hand"
2. Predictability of accentual mobility.
While it is not predictable from the standpoint of phonology, in the Romance languages, placement of accent, including it's mobility, can be predicted from the standpoint of grammar and lexicon. The rules for accenting Spanish (regular) verbs are quite simple, with each grammatical form having it's characteristic accentual properties, regardless of the conjugation class of the verb - some suffixes are unaccented, and others are accented. The same is true for derivational suffixes, some being accented, and others unaccented.
/ˈamo/ "I love" vs. /aˈmamos/ "we love" vs. /aˈmaba/ "I loved" vs. /aˈmabamos/ "we loved" vs. /aˈme/ "I('ve) loved" vs. /amaˈ re/ "I'll love"
/ˈtemo/ "I fear" vs. /teˈmemos/ "we fear" vs. /teˈmia/ "I feared" vs. /teˈmiamos/ "we feared" vs. /teˈmi/ "I('ve) feared" vs. /temeˈre/ "I'll fear"
/ˈparto/ "I split" vs. /parˈtimos/ "we split" vs. /parˈtia/ "I was splitting" vs. /parˈtiamos/ "we were splitting" vs. /parˈti/ "i('ve) split" vs. /partiˈre/ "I'll split"
This will be called a category-based mobile accent, because the grammatical category of a word (inflectional or derivational) usually predicts it's accent pattern - as can be seen above, 1. sg. present is accented on the stem, while 1. pl. present is accented on the person-number suffix, the imperfect tense is always accented on the tense suffix, while the preterite and future tenses are always accented on the person-number suffix, regardless of the conjugation class of the verb and the exact form of the grammatical morpheme.
On the other hand, in Slavic, the grammatical category of a word doesn't predict it's accent pattern, as can be seen in these three verbs in archaic Kajkavian, all belonging to the same conjugational class, but having different accent patterns (I've chosen archaic Kajkavian because it shows that not only the placement of accent, but also length and tone, can change, seemingly at random).
/ˈpaziti/ "to watch" vs. /ˈpâːzim/ "I watch/I'm watching" vs. /pâːzimo/ "we watch/we're watching" vs. /ˈpazil/ "he watched" vs. /ˈpâːzila/ "she watched" vs. /ˈpazi/ "watch out!"
/braˈniti/ "to protect" vs. /ˈbrǎːnim/ "I protect/I'm protecting" vs. /ˈbrǎːnimo/ "we protect/we're protecting" vs. /braːˈnil/ "he protected" vs. /ˈbrǎːnila/ "she protected" vs. /braːˈni/ "protect!"
/saˈditi/ "to plant" vs. /saˈdǐːm/ "I plant/I'm planting" vs. /sadiːˈmo/ "we plant/we're planting" vs. /ˈsadil/ "he planted" vs. /saˈdǐːla/ "she planted" vs. /saːˈdi/ "plant!"
Also, different morphemes expressing the same grammatical category can have different accentual properties, as can be seen in this list of Russian stress patterns of nouns - different cases have different accentual properties depending on the declension, and all patterns are equally regular. Something similar does exist in Spanish, where a group of preterites (so-called "strong preterites") has a distinct accent pattern from the one shown above, but these are considered irregular.
Such an accent system, where the accentual behavior of both roots (roots of same inflectional class can have different accent pattern) and suffixes (suffixes expressing the same grammatical category can have different accentual properties) is free from the grammatical category they belong to is called a paradigm-based mobile accent, because each word belongs to one of unpredictable (lexically specified) accent patterns, or paradigms.
This is the most salient feature of the Slavic accent system, and the one that baffles people the most (both professionals and non-professionals). In fact, Slavic languages are the only Indo-European languages that posses a mostly paradigm-based accent system - the accent system of Romance is mostly category-based (with the exception of irregular preterites), while the accent system of PIE, Greek and Vedic Sanskrit is mixed - it's, to simplify, paradigm-based in nouns, and category-based in verbs.
In the next post, I will show how to analyze the working of a paradigm-based accent system - such an analysis works on a typological level and can be applied to many unrelated languages, not just Slavic.
Feel free to comment and ask if anything isn't clear. I hope this grows into a fruitful discussion .