The usage of "It was" with "could we" in the following sentence seems intuitively wrong, but I don't know how to properly explain it:
"It was only after they were observed, could we explain certain phenomena."
These seem better:
"Only after they were observed, could we explain certain phenomena."
or,
"It was only after they were observed that we could explain certain phenomena."
And these seem better yet:
"Only once they were observed, could we explain certain phenomena."
or,
"It was only once they were observed that we could explain certain phenomena."
Could someone enlighten me as to the grammatical concepts at work here?