What are the differences between "и" and "і" in pre-reform orthography?

Question

In very old texts, sometimes the letter і is used, but it has been replaced by и in modern spelling.

I, like many others, learned modern orthography, so I don't understand why sometimes і is used, and sometimes и. As far as I know, they both represent the same phoneme, so I don't see what the motive was for having two separate letters.

Suppose I travel back in time, and need to know when to use which letter. How do I know when to use і, and when to use и?

Answer 1

The Cyrillic alphabet was originally the Greek alphabet, shoehorned into being a script system for Old Church Slavonic.

The original Greek alphabet had two distinct letters: ι (the iota) and η (the eta), which used to denote different sounds in Ancient Greek, but had later merged in Byzantine Greek (the source of sound meanings for the original Cyrillic).

Old Church Slavonic used both letters, but had later re-purposed them: i was used before the vowels, in some Greek and Hebrew names and loanwords which had the iota in their Greek rendition (стiхъ, Iиуй, Давiдъ etc.) and to resolve homonyms: миръ "peace" / міръ ("world, society").

Later, Russian orthography had brought the Greek and Hebrew loanwords to the norm, so the i was used:

1. Before the vowels and й, except on morpheme boundary: iудей, русскiй but семиугольникъ
2. In the word мiръ in its meaning of "world, society" and words derived from it.

That's it!

Answer 2

Unlike Ѣ (see "Ять в дореформенной русской орфографии"), the rules for i/и (which were pronounced exactly the same way) are very easy to memorize. Here's a quote from Wikipedia:

Употреблялась перед гласными (в том числе и перед й, которая считалась полугласной: кiй, убiйца), а также в слове міръ в значении «окружающий мир, вселенная» для отличия от слова миръ «спокойствие». Согласно народной этимологии так же писали Владиміръ, однако академиком Гротом предписывалось писать Владимиръ. Исключения составляли только сложные слова, первая часть которых оканчивалась на и: пятиаршинный, семиэтажный, восьмиугольникъ, а также приставка наи- (наиужаснѣйшій), отрицательные местоимения, начинавшиеся с ни- (ниоткуда) и т. п.

There was also the so-called ижица, which up to the beginning of the 20th century, was used only in мѵ́ро (миро) and a bunch of other words (for instance, ѵпостась, сѵ́мволъ). But this was dated even then.