Help with ESL tutoring

I'm tutoring a 9-year-old Korean girl in English. One of her biggest problems with English is using the definite/indefinite article. She often doesn't use one when she should. I'd appreciate some suggestions with how to help her with both conversational and written English.

[konungursvia]

I've taught Asian children this. Try telling her this: a means 1, any one. The means "a certain special one, you know which one, and I know which one."