Diacritics
Russian spelling uses fewer diacritics than those used for most European languages. The only diacritic, in the proper sense, is the acute accent ⟨◌́⟩ (Russian: знак ударения 'mark of stress'), which marks stress on a vowel, as it is done in Spanish. Although Russian word stress is often unpredictable and can fall on different syllables in different forms of the same word, this diacritic is only used in special cases: in dictionaries, children's books, or language-learning resources, on minimal pairs distinguished only by stress (for instance, за́мок 'castle' vs. замо́к 'lock'). Rarely, it is used to specify the stress in uncommon foreign words and in poems where unusual stress is used to fit the meter.
The letter ⟨ё⟩ is a special variant of the letter ⟨е⟩, which is not always distinguished in written Russian, but the umlaut-like sign has no other uses. Stress on this letter is never marked, as it is always stressed, except in some loanwords.
Unlike the case of ⟨ё⟩, the letter ⟨й⟩ has completely separated from ⟨и⟩. It is neither considered a vowel, nor even a diacriticized letter.
Read more about this topic: Russian Alphabet