Example:
What this new plan was I had no idea.
What was this new plan I had no idea.
What's the difference between the two? Is one more common than the others?
Example:
What this new plan was I had no idea.
What was this new plan I had no idea.
What's the difference between the two? Is one more common than the others?
What this new plan was, I had no idea. This is a statement, and a common way to say this.
"What was this new plan I had no idea." This is a question ("What was this new plan?") fused to a statement ("I had no idea."). It will be understood by most - along with the assumption that it comes from a non-native speaker. (US)
As the others have said, the 1st sentence is correct. It is a form of indirect (a.k.a. reported) speech, where the inversion that occurred after a question-word in a direct question has to change back to statement word order.
In direct speech you would have a question (inversion occurs):
A: "What is this new plan?"
B: "I have no idea."
Now B speaks about this conversation, and has to use indirect speech (word order goes back to: subject group-verb group).
I had no idea what this new plan was.
or
What this new plan was, I had no idea.