Technically, dandelions aren’t bad for your lawn, and they don’t directly kill grass. In fact, they appear in areas that are nutrient-deprived, and their long taproots can loosen the soil and bring nutrients closer to the surface. 

However, dandelions do spread quickly, and they can take over your lawn, eventually crowding out grass. This is why a healthy lawn is the best defense against dandelions.

No, it's not really possible to completely prevent weed growth. Weeds are incredibly resilient—all they usually need is a tiny bit of dirt to sprout, so their eventual appearance is pretty much inevitable. However, you can definitely keep them at bay by getting rid of weeds as soon as you notice them. The longer you let them sit and grow roots, the more they'll spread.

The most effective way to eliminate quack grass without killing the grass nearby is to overseed the area, keep the seed wet until it grows, mow often, and regularly fertilize the area to promote healthy grass that can choke out the invasive weed.

If possible, you should prioritize pulling weeds up by hand. However, be sure not to use your bare hands, as many weeds are sharp or otherwise irritating when they come in contact with skin. Always don thick, protective gardening gloves to protect your hands when you pull up weeds.