とても複雑な表現って英語でなんて言うの？

"That is an extremely complex expression."＝「それはとても複雑な表現です。」

"That is really difficult to put into words."=「それは言葉にするのがとても難しいです。」

Example sentence:
"Yeah, I'm not sure how to tell you this. It's not exactly easy to put into words."