When do you use シ instead of し?

In Japanese, the characters し (hiragana) and シ (katakana) represent the same phoneme but are used in different contexts.

Hiragana (し)

• し is used in hiragana, which is one of the two phonetic scripts of the Japanese language. It is used to write Japanese words, grammatical elements, and to phonetically spell out kanji characters.
• Examples include words like 死 (shi - death), 知る (shiru - to know), and grammatical constructions as described in the sources13.

Katakana (シ)

• シ is used in katakana, another phonetic script of the Japanese language. Katakana is typically used to write foreign words, loanwords, onomatopoeias, and to emphasize certain words.
• For example, シ might be used in words like シナモン (shinamon - cinnamon) or as an emoticon due to its resemblance to a smiling face1.

Summary

• Use し when writing in hiragana, which is more common for native Japanese words and grammatical elements.
• Use シ when writing in katakana, which is more common for foreign words, loanwords, and emphasis.

In the context of the character itself, whether it is し or シ, the pronunciation remains the same: 'shi' as in the English word 'she'1.