It is correct either way. I prefer it without "that"..
Student or Learner
"He suddenly realized that what he had done was really foolish."
Is it right to leave"that" there in the sentence? Shouldn't it be omitted? Thanks for help!
I agree it's correct either way. I prefer it with 'that'.
I prefer it with "that" solely because it tells the listener that the sentence isn't just going to be "He suddenly realised what he had done". That is a perfectly grammatical sentence but, by adding "that", we know that something more is to come after "what he had done".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.