Because the Cyrillic letter И was derived from the Greek letter Eta (Ηη), the Cyrillic ⟨И⟩ had the shape of ⟨Η⟩ up to the 13th century.
The name of the Cyrillic letter И in the Early Cyrillic alphabet was ижє (iže), meaning "which".
In the Cyrillic numeral system, the Cyrillic letter И had a value of 8, corresponding to the Greek letter Eta (Ηη).
In the Early Cyrillic alphabet, there was little or no distinction between the letter ⟨И⟩ and the letter ⟨І⟩, the latter of which was derived from the Greek letter Iota (Ιι). Both remained in the alphabetical repertoire because they represented different numbers in the Cyrillic numeral system: eight and ten.
In New Church Slavonic, they co-exist with each other with no pronunciation differences. But in Ukrainian and Rusyn, the two letters have different pronunciations. Other modern orthographies for Slavic languages eliminated one of the two letters in alphabet reforms of the 19th or the 20th centuries. The Russian, Macedonian, Serbian, and Bulgarian languages now use only ⟨И⟩, and Belarusian uses only ⟨І⟩. However, the letter ⟨І⟩ was also used in Russian before the Bolshevik reform of 1918.
Form
Originally, Cyrillic ⟨И⟩ had the shape identical to the capital Greek letter Eta⟨Η⟩. The middle stroke was later turned counterclockwise, which resulted in the modern form resembling a mirrored capital Latin letter N⟨N⟩ and so ⟨И⟩ is used in faux Cyrillic typography. However, the style of the two letters is not fully identical: in roman fonts, ⟨И⟩ has heavier vertical strokes and serifs on all four corners, and ⟨N⟩ has a heavier diagonal stroke and lacks a serif on the bottom-right corner.
In roman and oblique fonts, the lowercase letter ⟨и⟩ has the same shape as the uppercase letter ⟨И⟩. In italic fonts, the lowercase letter ⟨И⟩ looks like the italic form of the lowercase Latin U ⟨u⟩. Both uppercase and lowercase handwritten forms of the Cyrillic letter I look like handwritten forms of the Latin letter U.
Usage
Since 1918, ⟨и⟩ has been the tenth letter of the Russian alphabet, and in Russian, it represents /i/, like the i in machine, except after some consonants (see below). In Russian, the letter typically denotes a preceding soft consonant and so is considered the soft counterpart to ⟨ы⟩, which represents [ɨ]. However, unlike other "soft" vowels (е, ё, ю and я), и in isolation is not preceded by the /j/ semivowel. In Russian, the letter could be combined in the digraph ⟨ио⟩ (like ⟨ьо⟩, ⟨їô⟩ and ⟨iо⟩) to represent ё before it started around the 1950s, although that letter remains rare as people usually use е (apparent confusion has remained in the transcription of some foreign words).
⟨И⟩ was used significantly less in Russian before the Bolshevik reform of 1918:
⟨і⟩ was used before all vowels and before the semivowel ⟨й⟩ except at the end of a morpheme in a compound word, where ⟨и⟩ was used. So англійскій (English) used ⟨і⟩, but пяти + акровый = пятиакровый (five-acre) used ⟨и⟩.
⟨и⟩ was used as the last letter of a word and before consonants except in міръ for "world, universe, local community, commons, society, laity" (and words derived from it) to differentiate from миръ "peace"). After 1918, both are spelled мир.
According to critics of the Bolshevik reform, the choice of Ии as the only letter to represent that side and the removal of Іі defeated the purpose of 'simplifying’ the language, as Ии occupies more space and, furthermore, is sometimes indistinguishable from Шш.
⟨И⟩ is pronounced [ɨ] in ⟨жи⟩ (sounds like ⟨жы⟩[ʐɨ]), ⟨ши⟩ (sounds like ⟨шы⟩[ʂɨ]) and ⟨ци⟩ (sounds like ⟨цы⟩[t͡sɨ]), because in Russian, the sound [i] usually cannot be pronounced after "zh" ⟨ж⟩, "sh" ⟨ш⟩, and "ts" ⟨ц⟩.
In the Bulgarian Cyrillic alphabet⟨и⟩ is the ninth letter. It represents the sound /i/ and also occurs with a grave accent, ѝ, to distinguish orthographically the conjunction ⟨и⟩ ("and") and the short form of the indirect object ⟨ѝ⟩ ("her").
In Kazakh, ⟨И⟩ is used for /əj/ and /ɪj/ in native words and for /i/ in loanwords, and ⟨І⟩ is used for /ɪ/ in native words.
In Belarusian, the letter (и) is not used at all, and the sound /i/ is represented by the letter ⟨і⟩, which is also known as Belarusian-Ukrainian I.
The letter ⟨И⟩ is the eleventh letter of the Ukrainian alphabet and represents the sound [ɪ], a separate phoneme in Ukrainian. The Ukrainian ⟨и⟩ can be transliterated to other languages that use the Cyrillic script by either ⟨и⟩ and ⟨ы⟩ because of the lack of a uniform transliteration rule. Speakers of other Slavic languages can perceive Ukrainian [ɪ] as [i], [ɨ], or sometimes even [e] (see Ukrainian phonology for more on the pronunciation of [ɪ]). The sound [i] in Ukrainian is represented by the letter ⟨і⟩, just as in Belarusian.
American rapper Nathan Feuerstein is mainly known by his initials as "NF", which is stylized as "ИF".
Accented forms and derived letters
The vowel that is represented by ⟨и⟩ can, as is the case for almost any other Slavonic vowel, be stressed or unstressed. The stressed variant is sometimes (in special texts like dictionaries or to prevent ambiguity) graphically marked by the acute, grave, the double grave, or the circumflex accent.
Special Serbian texts also use ⟨и⟩ with a macron to represent long unstressed variant of the sound. Serbian ⟨и⟩ with a circumflex can be unstressed as well, which then represents the plural form of the genitive case to distinguish from other similar forms.
Modern Church Slavonic orthography uses the smooth breathing sign (Greek and Church Slavonic: psili, Latin: spiritus lenis) above the initial vowels (for tradition alone since there is no difference in pronunciation). It can be combined with acute or grave accents if necessary.
None of those combinations is considered to be a separate letter of respective alphabet, but one of them (⟨Ѝ⟩) has an individual code position in Unicode.
⟨И⟩ with a breve forms the letter ⟨й⟩ for the consonant /j/ or a similar semivowel, like the y in English "yes." The form has been used regularly in Church Slavonic since the 16th century, but it officially became a separate letter of alphabet only much later (in Russian in 1918). The original name of ⟨й⟩ was I s kratkoy ('I with the short [line]'), later I kratkoye ('short I') in Russian. It is known similarly as I kratko in Bulgarian but as Yot in Ukrainian.
Cyrillic alphabets of non-Slavic languages have additional ⟨и⟩-based letters like ⟨И̃⟩ or ⟨Ҋ⟩.