Nikkor 50mm with D3100?
Will this lens work with the D3100 > NIKKOR 50mm f/1.8D NIKON LENS
> what issues i may face with this lens if i use it with the D3100?
> Also what better. 35mm or 50mm for landscapes an group pics?
> Also if i have the 18-55 lens, will the 50mm be useful?
Yes it will mount & meter but you will not have autofocus - you will need to focus it manually.
For landscapes, you can use either - 35mm will obviously give you a wider field of view. For group portraits, you want the 50mm UNLESS you are space restricted, otherwise you may get edge distortion.
The 50mm is a faster lens than the 18-55mm. When you set you 18-55mm to 50mm, the widest aperture you'll be able to get is around f4. The 50mm f1.8 will be nearly 3 stops faster than this so will be more useful in low light conditions. It will also allow a shallower depth of field.
It will mount and it can work if you don't mind focusing it manually. It will not autofocus.
The 50 mm lens on your camera is a medium telephoto, not at all useful to shoot landscapes. The Nikkor lens you need for your entry level is the ultra-fast auto-focus AF-S 50 mm f/1.8. The older, slower AF 50 mm f/1.8 lens can't auto-focus with your camera (no internal focusing motor)
Before you invest in a wide angle lens for shooting landscapes, practice using your 18-55 at both 18 mm and 35 mm. You will soon see that the wider setting, 18 mm, is a better landscape focal length and will provide you with the angle of view you need to shoot groups.
For those who want to take brilliant landscapes, usually end up with a 10-24 mm lens.