Carpentry and Woodworking - Inside corner baseboards don't fit right

My hubby is putting down some baseboard in one of our rooms and although he figured out how to cut the joints to meet properly, in some of the corners if the ends do meet, then there is a big space left between them and the wall. He is letting this project just sit there so I need a solution for him so that he can finish with it. (Got more projects planned for him). Could someone tell me what he did wrong and how do we fix it?

Assuming this is painted baseboard, and assuming the gap is no more than 1/4", run the baseboard straight, trying not to bend it with the wall too much. When it is fastened, run a bead of caulk on top of the base and paint it. Quite often walls are not square, straight of plumb.

Assuming this is painted baseboard, and assuming the gap is no more than 1/4", run the baseboard straight, trying not to bend it with the wall too much. When it is fastened, run a bead of caulk on top of the base and paint it. Quite often walls are not square, straight of plumb.

It is MDF and painted, as instructions said paint before installation. It will get repainted again anyway. When his two ends are joined they are met perfectly however it is a big gap behind the corner to the wall. A bead of caulk won't fix that size of a gap. We tried to get the connectors but they were not available. Besides they don't look too good. If this is not crazy enough, I accidentally bought the wrong type of baseboard, its really door trim, sigh. I figured if we screwed this up we weren't out too much. but we have many more walls to do baseboards on and most are going to be of regular wood baseboard with a shoe kick added and this is more or less practice for the real wood. Not that it does not matter, but this is our computer room and nobody really cares about the corners. We just want to do it right and it bothers me that it hasn't been finished.

I assume these are miter joints. You may have him try "coping" the inside corners. You should be able to find the information on here or online in general with a little searching. Its a little more tedious but usually provides a better end product.

I assume these are miter joints. You may have him try "coping" the inside corners. You should be able to find the information on here or online in general with a little searching. Its a little more tedious but usually provides a better end product.

Thats what I told him I thought he might have to do, thanks for confirming that for me :) I just looked at the second corner under his desk and if the other end is already nailed then its just too short on the other side.