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I want to know if ¨'' it's not my lost'' is a correct grammar.. it is pertaning to a situation where your BF had another girl then you will say,, its ok.. he dont deserve me,, its not my lost! please help me to understand this.. thank you...
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You need the noun form: It's not my loss. (My loss is your gain.)

"Lost" can be an adjective: "Help me find my lost wallet."
It can be a verb: "I lost my wallet."
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I think this is more appropriate “He doesn’t deserve me. It’s not my loss.”

Maybe you don't realize it, but you are answering a question that was asked and answered many years ago.

Why not answer questions that are more recent? That way there's a better chance that the person who asked the question is still participating on our forum and can benefit from your answer.

CJ