You use this to refer to something very small, or an extremely small part of a whole.
Solo me diste una mirruña de tu sandwich, aun tengo hambre. /You only game me a small bite of your sándwich. I’m still hungry.
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I hope you have learned something new, about the spanish that real Mexicans talk on their everyday life.
If you want to improve your Spanish, I also give tutoring lessons on skype using this page that host many teachers from all over the world, and while I don’t really do grammar, I can help with conversation and doubts about the language. Also, if you are interested in any language besides spanish, there are teachers and tutors for any language there.