There are rules regarding what kind of sounds go together and where can they be found. Those rules are called 'Phonotactic rules' (or 'Phonotactic constraints'). If we used a single pronunciation for the -s endings in every situation, we would get ill-formed sequences of sounds, therefore we use three different sounds for the -s in order to conform with those rules.
- The S is pronounced [z] when the preceding sound is voiced because the sounds in the end of a syllable must agree in voicing, according to English phonotactics
- The S is pronounced [s] when the preceding sound is voiceless for same reason
- The S is pronounced [ɪz] when the preceding sound is either of [s, z, ʃ, ʒ, t͡ʃ, d͡ʒ] because two sibilants can't occur next to each other in the same syllable, so we insert a vowel ([ɪ] or [ə]) between both the sibilants to break that cluster
A syllable can be divided into three main parts: an onset, a nucleus and a coda. An onset is the consonant sound at the beginning of a syllable. A nucleus is in the middle of a syllable and usually a vowel. A coda is the consonant (or consonants) that follows the nucleus and is at the end of a syllable. So in the word dog /dɒg/, the /d/ is the onset, the /ɒ/ is the nucleus and the /g/ is the coda.
A sibilant is a fricative of higher amplitude and pitch and has a hissing effect e.g. [s, z, ʃ, ʒ]. A plosive is a consonant that involves the complete closure of the airflow such as [p, b, t, d, k, g]. A nasal is a consonant in which the velum is lowered and the air escapes through the nose e.g. [m, n, ŋ].
Obstruents are consonants that involve obstructing the airflow, fricatives, plosives, affricates are obstruents. Sonorants are speech sounds that are produced with continuous, non-turbulent airflow in the vocal tract e.g. nasals, liquids ([ɹ, l]), glides ([j, w]) and all vowels.
Affricates are [t͡ʃ] (as in chips) and [d͡ʒ] (as in jam). Geminates are lengthened consonants as in unnamed, solely etc.
Every language has a unique set of rules that determine the permissible sequences of sounds (i.e. licit and illicit sequences of sounds). That set of rules is called Phonotactic rules (or Phonotactic constraints).
In simple words, phonotactics studies what sounds go together and where can they be found.
A sequence of sounds that is allowed in one language may not be allowed in another language, for instance, the sequence /pn/ is allowed in Greek, but not in English. The p in the word pneumonia is therefore silent in English, but pronounced in Greek. Another example is /mbɑto/, which is a well-formed sequence in Swahili, but not in English.
Over time, a language may undergo phonotactic change, for example, the K in the word knight wasn't always silent as it is in Modern English, it was pronounced until Modern English.
English phonotactic constraints
There are loads of restrictions on syllable structure in Modern English, some of which are:
- No [ŋ] in the onset
- Obstruents in the coda must agree in voicing
- Two sibilants in the same syllable cannot occur next to each other
- No PLOSIVE + NASAL sequence in the same syllable
- No /h/ in the coda
- No tautosyllabic geminates
- No affricates or /h/ in complex onsets
and many more.....
Voiceless sound + S
Human vocal tract is designed in such a way that obstruents that disagree in voicing are hard to pronounce, because switching from voiceless to voiced consonants (and vice versa) requires independent movement of the larynx, which can be difficult to switch on and off at the millisecond timing required for consonant clusters.
When a word ends in a voiceless sound and we add the plural marker -s, the s is pronounced [s] in order to conform with Phonotactic rules. Some people claim that the original sound of the plural marker is /z/ and has three allomorphs [s], [z] and [ɪz ~ əz], anyway, when we add this to a word like bat; bat + [z] → *[bætz] — ill-formed, but when we change it from [z] to [s], then bat + [s] → [bæts] — well-formed.
Voiced sound + S
When a word ends in a voiced sound, and we add [z], then they agree in voicing and the combination is acceptable. For example, bag + [z] → [bægz] — well-formed.
Sibilants + S
When a word ends in a sibilant [ʃ], it's voiceless and when we add the [s] then we get *[ʃs] cluster, which isn't permissible, so we insert a vowel between both the sibilants in order to break that cluster and conform with the rules. After inserting the vowel, we get [ʃɪs], now we already said that the -s is [z] after a voiced sound, and the vowel is voiced, so we change the [s] back to a [z] and get [ɪz] therefore the word bushes is pronounced bush[ɪz]. When a word ends in [ʒ], we do the same as above.
[s] and [z] are sibilants, but I'm going to explain them separately. When a word ends in a [s], it's a voiceless sound, so we add the [s] form of the -s ending; bus + [s] → *[bʌss], here we have a geminated s and as we read in the rules that tautosyllabic geminates aren't allowed, therefore we insert an epenthetic vowel [ɪ ~ ə] to break the geminate: [bʌsɪs], we change the terminal [s] back to a [z] because the sound preceding is a voiced (vowels are always voiced): [ˈbʌsɪz].
The same goes for words that end with a [z]: when a word ends in a [z], it's voiced so we add the [z] form of the -s ending: rose + [z] → *[ɹəʊzz], here we have a geminated z, so we need to break it, therefore we insert a vowel: [ˈɹəʊzɪz]
Affricates—[t͡ʃ] and [d͡ʒ]—are complex segments. The second segment in both the affricates is a sibilant. So we get Sibilant + Sibilant, which isn't allowed. Therefore we insert a vowel between the affricate and the [s] or [z] to break that cluster. Beach + [s] → *[biːt͡ʃs] + [ɪ ~ ə] → [biːt͡ʃɪs]
It holds true for possessives and present singular -s too.
I've marked ill-formed sequences with a preceding asterisk (*)