mirror mod

sup guys, im new to blender and im having such an annoying start could someone please be of assistance
so i got a cube hit tab to open edit mode then shift r to split the middle from top to bottom , selected the left side deleted it then i opened the mods tab with the right side object selected and added the mirror mod thereafter the object was mirrored but when i try working with either side the opp side is not effected for eg if i try to extrude only one side will extrude and the other will stay untouched if anyone can please advise
Thank you

Don’t apply it.

Can’t offer more help without more info (screen shot or blend file)

Welcome to the forum.

Yes, surely you’re meaning the modifiers like temporary panels where to press “Apply” when you’re done with the setting. No, it’s not like that. Modifiers are often permanent during a project and they stay opened when you’ll open the file again. It depends on what you want to do and applying them it means that the effect you’re displaying is permanent. You should apply Mirror modifier only when you’ve done with modeling, so that when you’ve reached the final form you can make it permanent, and the two sides will become again independently editable (in their final form) and not related each other.
So, try to edit your cube with the Mirror modifier “opened”, and don’t press “Apply”. You’ll see that it will work. :slight_smile: Plus, settings in the modifiers can afflict the work in real time whenever you want, and they’re reversible (of course).