Hello!

I speak just a little english and sorry for my bad english, I need to know what "I have this thing for knives" means in english, someone told me that this means like "I love knives" but I think for this meaning would be "I have a thing for knives", can anybody help me? what it means exactly "I have this thing for knives"?

thank you in advance

You are correct in interpreting the line as "I love knives" or "I have a thing for knives."  It is the same expression, but colloquially.

The demonstrative pronoun "This" can be an informal substitute for the indefinite articles "a" or "an" in informal speech.

Thank you 12rainbow ;)

You are correct in interpreting the line as "I love knives" or "I have a thing for knives."  It is the same expression, but colloquially.

The demonstrative pronoun "This" can be an informal substitute for the indefinite articles "a" or "an" in informal speech.

 Quite a demonstration of vocabulary :) .

"I have this thing for ____" can also be taken as a rhetorical understatement of "I have an unnatural and/or obsessive interest in _____"  so that the person who's saying it would mean that they know their interest in the object is obsessive, derelict, or worthy of scorn, but that they are so attracted to it they don't care.

So yes, "I have this thing for knives" is a kind of tongue-in-cheek way of saying "I love knives," but also has an intonation of obsessiveness and even foreshadows some deviant ACT of what the knives will do in his hands.