* "an ear" is used when referring to a single ear, especially when it is indefinite or being introduced for the first time. For example, "He had an ear infection."
* "the ear" is used when referring to a specific ear, already mentioned, or when it is clear which ear is being talked about. For example, "He scratched his ear."
* "ears" is used when referring to multiple ears. For example, "The dog has floppy ears."
Let me know if you'd like an example sentence using "ear" in a specific context!