"Where Do Sentences Come From?"

Same place that ideas come from, of course: Schenectady. (That's what science fiction writer Harlan Ellison used to say whenever anyone asked him where he got his ideas.)

Essayist Verlyn Klinkenborg has a more erudite and complicated answer about the origins of sentences in this New York Times "Draft" column:

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Now try turning a thought into a sentence. This is harder than it seems because first you have to find a thought. They may seem scarce because nothing in your education has suggested that your thoughts are worth paying attention to. Again and again I see in students, no matter how sophisticated they are, a fear of the dark, cavernous place called the mind. They turn to it as though it were a mailbox. They take a quick peek, find it empty and walk away.

 Klinkenborg offers a few notions about how to become more comfortable with that scary dark place so that you can turn your interesting thoughts into interesting sentences. (Note: The New York Times has a paywall, but you can read a certain number of articles for free each month, so choose wisely. "Draft" is usually worthy of your clicks.)