What is the difference between osteopenia and osteoporosis? what effect does nutrients like calcium and vitamin d have on these conditions?

Matter of degree: Osteopenia can simply refer to loss of bone substance (density) on x-rays. It can also mean bone loss not severe enough to be called osteoporosis. Osteoporosis is density more than 2.5 standard deviations less than normal young people (t score less than -2.5). Osteopenia is T-score of -1.0 to -2.5. Calcium and vitamin d can help, but check with specialist. Other conditions can mimic osteoporosis.