When to use fresh. Usually fresh seasonings are best, not always convenient or inexpensive, but certainly the best tasting. There will be certain recipes that require fresh seasonings. One would be pesto. Another might be an Italian salad where you are suppose to actually see the basil leaves ... the "flakes just won't cut it. Again, fresh herbs almost always enhance your recipe and taste better. Could still get similar results using a dried version, well sometimes depending upon the recipe.
When to use dried. A good example of this would be in the preparation of a salad dressing where the dried herbs become saturated via the oils is a good example. Another good example of the use of dried seasonings is in the preparation of dry rubs that are applied to the meat. Meat has natural moisture and that allows the dried spices to become moist and saturated. Besides, it would be hard to store fresh seasonings in a dried rub.
When you can choose between the two. If you really want to choose one over the other, what is the ratio? Well it sort of depends. We use a ratio of 3-4 parts fresh equals 1 part dried. Sometimes
you may want more or less. Another convenient conversion would be 1 Tablespoon equals 1 teaspoon.
Good luck and have fun!