The rule is, that it is incorrect to use myself in a sentence that does not have an 'I'.
The word myself is a pronoun, it can either be an emphatic pronoun or a reflexive pronoun.
I did this myself - here 'myself' is an emphatic pronoun because there is an emphasis on how you did something. Similarly, I pricked myself with a pin - here 'myself' is a reflexive pronoun. When 'myself' is used to show that you are doing something to yourself it is reflexive.
'Myself' cannot be used for any other reason. It cannot be interchanged with I or me. It cannot be used when someone is doing something to you.
While giving an introduction:
Many people tend to introduce themselves as: "Myself Rohit" (this is incorrect). The correct way to introduce would be: "I am Rohit".
'Myself' cannot be used without an 'I' in the same sentence and for the same reason saying "myself stepped on the puddle" would be incorrect. "I stepped on the puddle" will be correct.
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