In the movie "My Fair Lady", the main character will use this structure a lot; "I'm a good girl, I am." or "I washed my face and hands before I come, I did.". I like to know more about this structure and when such thing can be used. Thank you for your help.
This repetition acts much like an exclamation mark to insist on the truth of what has gone before.
It should not be emulated, unless you are aiming at a comic effect. Note that Eliza Dolittle comes to Prof. Higgins to lose her vernacular usage:
That gentility, as she learns in Act III, is not merely a matter of pronouncing words properly. She must also master the terms and phrases and syntax of "proper" English. One of the vernacular tricks she loses is just this insistent repetition: it disappears after Act II and does not return until Act V, at the precise moment when Eliza realizes her true strength and taunts Higgins by deliberately reverting for a single sentence to her old way of speech:
That sentence is the climax of the entire play, but the form of expression is ironic. I presume that you, like Eliza, want to employ "proper" English, not the street speech of 1913. If so, avoid this use.
The quotations above are from the text of Shaw's Pygmalion; I don't know whether they're preserved in the script of the musical and the movie.