Modifying a Duplicate Layer/Layer Mask

[festus]

Hello,
I can't seem to get my head around layer masks and modifying layers.

I opened a .CR2 (canon raw file) as a layer, made a duplicate layer.
Tried various tools on the duplicate but they did not effect the image.

Next I added a layer mask to the duplicate, tried various tools on it and received the same results - no modifications.

If I tried various tools on the initially loaded .cr2 file, any changes I made took effect.

This is my first experience at modifying images so I have no previous experience.

I am looking for advice as to what I am doing wrong, please.

Thanks,
dlb
.

[Ofnuts]

When you add a layer mask, all changes you do next apply to the mask and not to the layer (until you explicitly reset the layer th0umbnail). Changes in the layer mask only change the opacity of the layer. Since the layer you work on is a copy of the layer under it, when you make part of it transparent you see the layer below but since it is identical you don't see anything changing.

[festus]

Ofnuts, Thanks for your reply. It has taken me awhile to understand layers and masks but with your reply and the video from GuruGal (https://www.youtube.com/watch?v=qPMmfUNmsJM), I think I am understanding how all of this works together.
Thanks,
dlb
.