When I read a paper about graph cuts, I found some sentences which were hard for me to understand. Graph cuts are partitions of the vertice of a graph. It's very complicated so I'm not surprised you might find it difficult to understand!!!

What is "graph cut"? Since it means nothing, the rest of the sentence is moot.

"When I read a paper about graph cut, I met some senteces hard for me to understand." "When I read a paper about graph cut(tings), I (found) some sente(n)ces (a bit) hard for me to understand." <---- this is perfect english and is less awkward. I don't know what a "graph cut" is, so you might be giving an incorrect name. if you want to sound more native, you can replace "hard for me" with "difficult", but they are both grammatically acceptable.