Writing system · Foundation
Katakana long vowels and ー
How the long-vowel mark works in loanwords and names.
In short
The mark ー extends the vowel sound of the preceding katakana syllable.
How it works
In コーヒー, the first ー extends コ and the second extends ヒ. The exact spelling of loanwords is conventional, so learn common words as written rather than trying to rebuild them from English spelling.
Useful contrast
Hiragana usually writes long vowels with vowel kana rather than ー in ordinary words, although stylistic exceptions exist.
Examples
コーヒー
coffee
スーパー
supermarket
Common pitfall
Do not read ー as a dash or pause. It belongs to the pronunciation of the word.