Boolean Modifier cutting the wrong part

Question

So, I have been trying to make a Jett's knife model in Blender. Everything was fine until I tried to cut a hole in the knife. Since I have only been using Blender for like 5-6 days only, I searched for some ways to do that. I found that I needed to use a Boolean Modifier, so I did. I selected the knife, and the cube that I edited into the shape of the blade. But, when I selected the cube in the "Object" part of the modifier, it removed the blade instead. I don't know why this happens, maybe I did something wrong. I tried to follow other things on the Internet but it still failed. How can I fix this?

I was expecting the circled part to be cut out. But it cut the outside instead.

https://youtu.be/qV_g0ASsFHo Here is what I did

Answer 1

I've had a chance to look at the actual blend file, and I found geometry problems. Let me show you how I found and fixed one.

I took your Cube cutter into edit mode in a local window. (If you have a number pad on your keyboard the / character on the pad toggles that mode.)

Then I used the select menu to select interior faces

Turns out you've got one:

That's going to confuse the boolean operator, so delete it. Use the X key and select "Faces".

It still doesn't work because you have two objects you need to apply the Boolean to, "Blade" and "Middle Knifu"; so I added the boolean to each.

and it worked.

ASIDE: You have a lot of bad geometry in both the blade and Middle Knifu. In addition to interior faces you have duplicated vertices and you have interior edges.

ORIGINAL ANSWER STARTS HERE:

Be sure that you apply the Boolean modifier to the cube that represents the blade and not the object you are using to 'cut' the hole. Here's a crude example:

What you don't see in the example is the cylinder I used as the 'cutter' object. If you look at my scene collection you'll see why:

When you select the blade and go to the modifiers panel, the Boolean modifier should look like this:

Notice that the modifier is applied to the blade and that the "Object" in the modifier is the cylinder.

Below the Object there's a 'Solver' box. I have it set to Fast.

A common rookie mistake is to apply the modifer but leave the cylinder visible. If you do this it looks like nothing happened:

There are a couple of ways to solve this. For the example I simply made the Cylinder invisible:

If you do it this way, the cylinder will still show up in renders. You have to also make it invisible to the camera:

Alternatively, if you are going to use a lot of Booleans a better technique is to move the cylinder to a new collection, usually called 'cutters':

Select the cylinder and type M. this will bring up a menu:

Since you don't already have a collection, select "new collection" and give the collection a name.

Now all you have to do is uncheck the box for the cutters and the cylinder won't show up in the 3D view or on a render: