Hello

I'm a bike newbie and having a problem with the Shimano RD-2300 (rear) derailleur and the Shimano Revoshift SL-RS41-8A twist shifter available on my Birdy World Speed :

1. I need to shift two gears to move from 8 to 7 (ie. smallest/fastest gear to second), since the derailleur doesn't actually move when I turn the shifter. Same thing happens between 6 and 5. FWIW, the sprocket is a Shimano 8-speed CS-HG40.

2. I can no longer go below 5 (ie. fourth ring from the right), so speeds 4-1 are unavailable.

3. It's a minor thing, but the 8 number is no longer displayed when I shift back to it. It seems to be stuck within the ring, and only the blank, black space between 7 and 8 is shown . I don't know if it means anything.

4. Generally speaking, I don't know what to do with the two screws at either end of the cable, how to adjust the cable itself, and whether this issue I'm having requires adjusting the H and L screws. So many options, so little time :-)

I'd like to learn to adjust this myself to 1) avoid going back to the store and paying $50 just for this and 2) be able to fix this on the road if the same issue pops up again.

Thanks for any help.