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Which is correct, A or B?
A. What do you think is the most important thing?
B. What do you think the most important thing is?
Somebody said A. But I don't know why.
Thanks for your help.
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Both are correct.

Rommie
Comments  
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Hi Rommie,

I have no objection at all to your answer because both are well received in every day usage. Though I am far from a purist, it may help us if we spend a little more time to think about the possible difference between the two sentences.

A. What do you think is the most important thing?
B. What do you think the most important thing is?

If we consider “do you think” in both sentences as a parenthetical, we would vew them in the different perspective. Then, the remaining sentences will read for each as

A. What is the most important thing?
B. What the most important thing is?

As an interrogative sentence, A looks better than B.
Regardless, I think both are decent enough as far as the acceptability is concerned.

I am always appreciative of your contribution on this board, Rommie

Mirapence
Thanks Mirapence. I do agree with you.

It could even be argued that they mean different things. After all, the following do:

1. Tell me what is green.
2. Tell me what green is.

Rommie
Some more examples:

1: What's going on?
2. Tell me what is going on.
Students: Are you brave enough to let our tutors analyse your pronunciation?
Thank a lot, Rommie. It's a nice pair of examples. I may need them for a future use.

I think the first "what" is a relative pronoun, and the next one is an interrogative pronoun.

Please feel free to comment on my answers to questions if they need any additional clarifications. Thanks.

Mirapence