"This I know today, but back then..."
What does this word order show? What's the rule?
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In "This I know" there is no subject-verb inversion. Inversion would be This know I or better, as Jonathan says below: This do I know.
"this" is placed at the beginning of the sentence which is called fronting. The word in unusual position gets more emphasis or dynamic.
You should change your headline. Maybe: This I know. Inversion or fronting?