Vicente,
As always Mer has done a wonderful job answering your questions, but I think you'll find that sencilla in "soy una chica sencilla" does not mean single. Sencilla means simple or uncomplicated. It's used as a positive term in Mexico meaning someone who is not snobby or arrogant.
It's interesting that young Mexicans use antrear much the same way American young people have turned the noun club into a verb by saying "let's go clubbing".
Regards,
Joel
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