I try to add a mirror as modifier, but it just doesn't work and I don't know why please help me, I start a new file then go unto Add Modifier and choose Mirror, change nothing and click apply but nothing happens
The Mirror modifier mirrors the object about it's origin and merges the vertices that are within the
Merge Limit of each other.
If you've just started a new session with the default Cube, then the cube's origin is in the center of the cube. If that is your setup, you aren't going to see anything interesting unless you either:
- move the origin of the cube elsewhere: try moving the cube object, then hit Ctrl+A > Apply Location
- or start editing the actual mesh itself in Edit mode (select cube, hit Tab and start moving vertices around.)
If adding modifier after these actions you'll see some difference.
The aforementioned suggestions aren't really a good use of the modifier, but they'll help you see that it's actually active and working on your scene. If you are wanting to look at a modifier to see what it does (or what multiple instances of it might do), may I suggest you take a look at the Array modifier?