Student or Learner
During the class, I have found a little strange sentence.
"By 1985, the amount of energy needed to produce a ton of grain had more than doubled."
I have never seen the sentences that some words are put between 'had' and 'participle' such as 'more than' in the above sentence.
Is it right grammatically?
Yes it is correct.
Starting from the basics it looks like this:
(By 1985) the amount had doubled.
Everything else is filler. It increased by more than two times so it more than doubled.