* "Used" is the past tense of the verb "to use." It indicates that something was used in the past.
* Example: "I used to live in New York." (I lived there in the past, but I don't live there anymore.)
* "Not a" is a negation. It means something is not a particular thing.
* Example: "This is not a dog, it's a cat."
When to Use Each
* "Used" is used when describing something that was used previously.
* "Not a" is used to contradict a classification or identify something as being something other than what was assumed.
Examples:
* "Used":
* I used to play the piano.
* The car was used for deliveries.
* She used to be a teacher.
* "Not a":
* This is not a house, it's an apartment.
* He is not a doctor, he's a lawyer.
* That is not a book, it's a magazine.
Let me know if you have any other questions about grammar!