I have the most difficult time translating my thoughts into coherent language. While there's a jumble of words bumping into each other in my head, the process of stringing them into sentences takes me longer than it would for most people. As a result, I could begin a sentence only to pause before I finish it (embarrassing, I know). With writing, I could at least complete my thoughts without making it look like I just trailed off for no reason.
I like Naturalist's idea of diagramming-I use a similar method before I talk on the phone, so I'd have something to refer to while speaking.
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“Oh - You're a very bad man!
Oh, no my dear. I'm a very good man. I'm just a very bad Wizard.”
― L. Frank Baum, The Wonderful Wizard of Oz