"You do not want to think about him at all."
Translation:Vůbec na něho nechceš myslet.
All of these are strange. Maybe the second one sounds closest to acceptable, but still strange. Why? I don't know, it just sounds strange.
The basic Czech word order is SVO, but is very flexible. But not completely free.
It means "na něho" or "na něj" (ie. the preposition "na" + personal pronoun "on/ono" in the accusative, singular). It is archaic/bookish and used very rarely nowadays. It's accepted just in case someone knows the word.
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Please not that this sentence is particularly challenging with a large or even huge number of possible forms. Many are still missing and some are less natural and it is not that easy to decide which ones to accept. It is so complicated that there is no way to work on it from my present location without a computer.