Can i get back a stolen cs;go knife if items were exchanged in the process?

No.. you accepted the trade. If he had charged back money or something, then you would have right to contact support and let them restore items.

There's a reason a big pop-up window asks if you are sure.

There's also a reason that when someone changes the deal, you get notice of it.

Sucks that you were scammed, but next time only deal with reputable buyers and pay attention.

Why did not not check before doing the trade,i always check the email to make sure what i selected to trade is what is being traded....you can try and contact Steam but i am not sure in this case they will do anything.

Last i knew if you get scammed your on your own now for the most part

how did it happen anyway?
Any steam trade involving a single item worth more than $5 require you to confirm the trade via email.
For the looks of things, you're the "idiot" here. no offense.
Trade is final.
Tough luck, mate.

well i will have to pay with my own wallet now,just great.....worked a month to waste 15% of my paycheck

It's your own fault. There's so many confirmation steps now, if you were 'not looking for a second' then you must have turned them off.
I wouldn't even call this a scam.