Mirror Modifier issue

So I am a total Blender noob, and I’m having an issue with the mirror modifier. I have a subdivided cube that I mirrored but it is not ‘connected’ to itself like all the other objects I have mirrored and it’s really messing with me. I tried to upload pics but it won’t let me. Basically I grab the non-mirrored section and if I move it along the Y-axis (which is the axis I oriented the modifier) the cube splits apart from itself.
Thanks

Check if the vertices you try to move are over the middle of the mesh or maybe you’re try to modify the modifier and not the object, disable the modifier to better understand what you’re doing

It will be because when you move the object in edit mode the origin point is not moving but geometry is and because you dont have turned on modifier visibility in edit mode it looks like two cubes are there. Or at least I guess from what you have wrote.