It has a function where, when the room gets dark, the back light will turns on. It has a function where, when the room gets dark, the back light will turn on.

Therefore even when the room is dark, I can still see the time. Therefore even when the room is dark, I can still see the time.

The back light isn’t strong, but it still makes the room becomea little brighter. The back light isn’t strong, but it still makes the room a little brighter.

After almost three months, when I changed the battery, I notice there is a switch aton the top sidethat can turn off that function. After almost three months, when I changed the battery, I notice a switch on the top that can turn off that function.

(Stupid! ) (Stupid!)

This doesn't really make sense in this context. Maybe write, "I thought it was stupid of me never to have noticed that."

I was happy the back light iwas off and the room iwas dark. I was happy the back light was off and the room was dark.

After another three months, I changed the battery again. After another three months, I changed the battery again.

But this time, I forgot not to turn off the back light. But this time, I forgot not to turn off the back light.