What did he do that was so wrong?
Why don't you say that without 'that'? I asked this somewhere else a few months ago, and got an wonderful answer. I thought that was reasonable enough. The answer said that you always need a subject for every verb, and if you take away the 'that', then the sentence would lack the subject for 'was'.
But see this example below.
Who do you think work the most?
Here, doesn't 'who' work as the subject for 'work' properly? Why do natives say this without 'that' when you say the very first sentence always with 'that'?