I am trying to replace a regular switch with a dimmer switch (single pole). There were two wires connected to the old switch (besides ground). The first is 2 black wires twisted together with a single red wire coming off it. The second is a black wire is live (has red tape). I first tried connecting the red wire with the 2 connected black wires to one of the black wires on the switch. And I connected the live black to the other black wire on the switch. The dimmer part of the light switch worked, but I was unable to turn off the light switch. What did I do wrong? Do I just need to try switching them or is there more to this? (I connected the ground to the ground wire in the box.) See photo.
It sounds like (from the comments) that you have a mis-wired three way switch. What I bet is happening is that the blank switch has a traveler shorted so that when you turn the dimmer off, it’s sending power over a traveler and still lighting the light.
To fix this, you’re going to have to identify the four wires that make up the three way circuit and fix the problem. You need to find: the incoming hot, two travelers, and the hot that goes to power the light fixture. Normally one box will have the incoming hot and the other will have the hot for the light, but sometimes one box has both and the two boxes have three wires shared between them (normally only two travelers are shared). I would guess the two wires you have nutted together in the picture are the travelers, but we’d really need to see what’s in that blanked out box.