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"The ideal would be that everybody WOULD SAY his opinion frankly."

Is this sentence correct?

My doubt concerns chiefly the construction with "would" (I mean the second "would", "would say"): is it right?

And would it be possible to use also other constructions? For instance:

"... that everybody SAID ..."

or

"... that everybody SAYS ..."

or even (perhaps only in a very formal, literary style?)

"... that everybody SAY ...": are these sentences correct?

Many thanks for your explanations.
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It might be better to say "state his opinion" rather than "say his opinion".

I would probably generate:

"The ideal would be for everybody to state his opinion frankly."

or "The ideal would be if everybody stated his opinion frankly."

or possibly, "The ideal would be if everybody were to state his opinion frankly."

I would not think the following incorrect, but strange, in spite of my love for the subjunctive!Emotion: smile

"The ideal would be that everybody state his opinion frankly."
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I see. Thank you.

And what about the other alternatives: "... that everybody WOULD STATE ...", "... that everybody STATED ...", "... that (if?) everybody STATES ..."? Would you consider them to be ungrammatical (not English)? Or rather awkward, clumsy? Or what?
By the way: what about "to TELL one's opinion"?
Briefly, all the other alternatives under consideration here are a little awkward, although completely understandable. I don't know whether they are, strictly speaking, ungrammatical (from the point of view of a grammar book), but if they are, I'm sure someone will notice and post a reply!

As for "tell", that won't work because "tell" needs to be used with the person you're telling, e.g., tell us, tell me, tell my friends, etc.
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