Translation:That is not definitely a knife. This is definitely a knife.
It does seem an odd wording in English. I'm sure the reason is that in the Klingon the "not" is attached to the verb ("it is"), but the "definitely" is attached to the noun ("a knife"). I suppose another way to express that would be, "it is not a definite knife." In fact, that might be a better way to translate this Klingon sentence, because it more closely connects the "definitely" and the "knife". In English, that sentence also sounds a bit odd, though perhaps a bit better. The most natural English sentence would be, "it is definitely not a knife," though I would normally expect that to be taj 'oHbe'bej.