I think him to be rich.

Isn’t this a pretty informal way to express the idea?

It sounds rather outmoded and quite formal to me. Today you’d more likely hear something along the lines of

I think he’s rich.

or

He seems pretty well-off to me.

depending on the context of the phrase.

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I does make sense but it sounds like a clumsy way of saying it.

I agree with ananaso, that most people would simply say ‘I think he’s rich’.

However, if you wanted to keep a similar sentence structure to the original, most people would say ‘I consider him to be rich’ (more formal) or ‘I think that he’s rich’.

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