Diacritic
In most other cases, use of diacritics for native English words is considered old-fashioned (not used anymore). Diaereses (similar to umlauts) can be used on words where two vowels next to each other are pronounced separately like noöne, reëstablished, or coöperate (two vowels pronounced together are a diphthong). This method is still used sometimes. Diacritics are sometimes used in loanwords (words of foreign origin), such as  naïve, entrée, pâté, façade, which are French words.