Here's why:
* Pronouns refer to nouns. A pronoun takes the place of a noun that has already been mentioned. This noun is called the antecedent.
* Clarity is key. Starting a sentence with a pronoun without an antecedent creates ambiguity. It leaves the reader wondering "Who is this pronoun referring to?"
Example of a sentence with a pronoun without an antecedent:
* Incorrect: "It was a beautiful day." (What is "it" referring to?)
Example of a sentence with a pronoun with an antecedent:
* Correct: "The sun was shining brightly. It was a beautiful day." (The antecedent of "it" is "the sun")
To start a sentence with a pronoun, you must first introduce the noun it refers to.