When c is followed by e, i or y, it usually says /s/ — like in city, cent and cycle.
The rule
C commonly represents /s/ when the next letter is e, i or y.
What is really going on
Palatalisation before front vowels, inherited from Late Latin through Norman French. Reliability is high in word-initial and word-final position. The main breaks are soccer and Celt, and words where the c is followed by an e that belongs to a different morpheme.
Why English does this
In Latin, c was /k/ everywhere. In Late Latin and then Old French it softened before the front vowels e and i, and English borrowed the words after that change had happened — which is why the rule exists at all and why it holds so consistently in words of French and Latin origin.
Position in the word
Beginning of a word — Reliable. For example: city, cent, circle, cycle.
Middle of a word — Reliable. For example: pencil, decide, recent, bicycle.
End of a word — As -ce, reliable. For example: face, race, nice, dance, ice.
When this rule works
The letter directly after c is e, i or y.
When this rule does not work
The next letter is a, o, u or a consonant — then c is /k/.
The word is soccer or Celtic, the two common exceptions.
The c is part of ch, which is a different grapheme entirely.
Made-up words that follow the rule. A child who reads these has the pattern rather than the vocabulary.
cith, cesp, brace
Exceptions
c stays hard before e or i: soccer, Celt, Celtic, sceptic. A short list, mostly recent or non-Latin borrowings.
Both sounds in one word: circle, circus, success, accident. The rule applies twice, correctly, to two different c's.
Contrast pairs
city versus cat — Soft before i, hard before a.
cent versus cot — Soft before e, hard before o.
cycle versus cup — Soft before y, hard before u.
race versus rack — The e keeps the c soft; the k makes it hard.
Decodable sentences
The mice raced across the ice.
I can see the city centre from my place.
Use a pencil to circle the price.
Common reading mistakes
city: a child reads /kɪti/, where the word is /sɪti/. Hard c applied before a front vowel.
cat: a child reads /sæt/, where the word is /kæt/. Soft c over-generalised after the rule is taught.
Do not teach it like this
"C always says /s/ before e, i and y."
Soccer and Celtic are both words children meet, and absolute wording makes a child distrust the whole rule the first time it fails. The rule is strong; it is not a law.
"C commonly says /s/ before e, i or y." Then show soccer, so the child meets the exception with you rather than alone.
Teacher note
Teach soft c and soft g together — the condition is identical and the pairing halves the memory load. Sorting is better than telling: give a mixed pile of cat, city, cot, cent, cup, cycle and have children sort before you name the rule. Do not introduce it before children are secure with hard c, or they will over-apply it to cat.
Teaching sequence
Confirm hard c is automatic: cat, cot, cup, clap.
Sort a mixed pile and let the children find the pattern themselves.
Name the rule with hedged wording, not absolute wording.
Add soft g the following week, with the same sort.
Introduce soccer as the exception, deliberately, rather than letting it appear by accident.