There are different Usages of the letter I.
1. the ninth letter of the English alphabet
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
This letter denotes the next after H in a set of items, categories, sizes, etc. e.g. It also denotes the ninth of ten or more hypothetical people or things. It is used by a speaker to refer to himself or herself. I = third Vowel.
2. Roman Numeral
This I equals to One in roman numerals.
I = Iodine and I = Electric Current. Besides, I letter of English Alphabet is used in Italy as the International Vehicle Registration.
I = Iodine, the Element in the Periodic Table with atomic number 53, that is a non-metallic element which forms black crystals and a violet vapour. A solution of Iodine in alcohol is used as a mild antiseptic. In Physics I = Electric Current in formula V=I/R where V is the potential difference and R is the resistance of the conductor.
The letter I denotes self-consciousness or Ego.
The most Common Noun which is used to learn this letter at the Nursery level is Inkpot; I for Inkpot.