I must always be capitalised, whether it's at the start of a sentence or not. There's no real reason why it is capitalised, but it's been that way for around 700 years and so is now the only accepted way for the pronoun to be written. This EL&U question offers possible origins of the capitalisation of the pronoun
If the letter
I is not being used as a pronoun, standard capitalisation rules apply:
- If the letter appears at the start of a sentence:
Individual writers write individually.
- If the letter is the first letter of a proper noun:
The population of India is around 1.3 billion.
- If the letter is part of an initialism or acronym, unless that initialism
or acronym has become so ubiquitous that it is treated as a word rather
than an acronym/initialism:
In 1908, the FBI was founded as the Bureau of Investigation.
Zip [Zone Improvement Plan] codes are a form of postal code in the