Which sentence is correct?

"All I am able to comprehend is that my brothers were right."

or

"All that I am able to comprehend is that my brothers were right."

2 answers

Both are correct. However, the second one is clearer.
1st one, because accdg to the book i've read, "that" is an essential clause we use. so it's like all I am able to comprehend is what? For ex. My bag "that" my mother bought has been stolen by him. My bag is what? we use "that" to introduce the subject or if it is still unknown! :D