Crabgrass may look green and fill in bare spots in the yard, but it is a weed that will take over your lawn in time, creating an unsightly yard. There are ways of killing the weed and restoring your lawn's grass. You will have to be diligent to succeed, and it might take several attempts before it is all gone. Try more than one of the methods it if the first doesn't work.
Spray vinegar directly on sections of crabgrass if you have large patches. Vinegar will kill the weed. You will need to fertilize and reseed the area after you kill the grass. Pull up the dead weeds before reseeding.
Put dark plastic over large patches of crabgrass. Blocking light from the plant will kill the weeds slowly. It might take a week or more. Put rocks on the plastic to keep it from flying off.
Apply a herbicide that kills weeds and not grass. This is called a selective herbicide. It will work as long as you can get the chemical to stay on the crabgrass until it is absorbed. With sprays, this is easy. The spray will land on the blades and dry there. Your only concern with liquid herbicides is rain. Schedule your use for when rain is not in the forecast. Granule herbicides don't necessarily stay put. They roll or are blown by a gust of wind, and there is no way to keep them in place. For this reason, if possible, you should use a spray formula.
Plant a grass that chokes out weeds, such as centipede. This type of grass slowly covers a lawn in runners of grass blades that cover and push out all other grasses and weeds.
Cut into the root of the weed and then pull it out by hand. Do this a little at a time each day, and before long you will be rid of the nuisance.