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"Sir" is a form of "sir", a noun which is often translated as "el señor". "Mr." is an abbreviation which is often translated as "Sr.". Learn more about the difference between "sir" and "Mr." below.
sir()
A noun is a word referring to a person, animal, place, thing, feeling, or idea (e.g., man, dog, house).
Mr.()
An abbreviation is the shortened form of a word or group of words (e.g., "UFO" = "unidentified flying object"; "p." = "page").
1. (mister)