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Soft sign The letter ё in Russian is optional, it is allowed to be replaced with е. Using ё is recommended for dictionaries, kids’ books, books for language-learners. In Wikipedia, there is a rule to use ё consistently in all articles. However, in most other texts, ё is used irregularly or not used at all.
The е/ё merger in writing does not make a major problem for an experienced reader since the meaning of the words differing only by е/ё (все ‘everybody’ — всё ‘everything’; осел ‘has settled’ — осёл ‘donkey’ etc) is usually clear in context. This however can be an issue with some exotic words, such as proper names, the correct pronunciation of which can be unknown to the reader. The same is true for the stress in words which is not normally marked, but can be marked in dictionaries, in exotic words, or in situations of ambiguity. The ё letter, however, is used much more often than stress marks.
The other Russian letter with a diacritical mark—й—is never substituted with и. It is obligatory.