The old self-deprecating remark that "I can't drive in a nail straight" is true as often as not. Lots of people can't. The reason is that a hammer seems like the simplest of tools to use and, when it fails to accomplish its purpose quite easily, its user feels a bit helpless.

Actually, a hammer IS a simple tool to use - if it is handled properly. The hammer should be grasped near the end of the handle so that you get full leverage when you strike a blow.Use your wrist for light taps, your arm and elbow for medium blows and your full arm, including your shoulder, for heavy strikes. Hit the nail or whatever squarely. Avoid glancing blows and under strikes or over strikes.

To drive in a nail straight, rest the face of the hammer on the nail, lift it just a couple of inches and tap the head of the nail. Holding the nail with your free hand, hit it a few more taps.

Once it has set solidly into the wood, release your free hand and complete the action. If you are driving the nail into hardwood, a pilot hole should have been made to accept it, a move that helps prevent any bending of the nail. Even in softwood, it's a good idea to pierce the wood a bit with an awl or similar pointed tool.

If you have only one hammer in the house, it should be a medium-weight one of 16 ounces, although some people prefer a light hammer. The important thing about selecting a hammer is that it should feel right in your hand as you pick it up and manipulate it with your hand and wrist.

In any case, the 16-ounce hammer with a curved claw can handle the many household projects requiring the use of the relatively soft common nail. A curved claw gives more leverage for removing nails, while the straight claw is best for ripping and tearing pieces of wood. This type of hammer should not be used for masonry nails, cold chisel or other metal.

A medium-weight ball peen hammer is for striking the very small hand chisels and punches for riveting, shaping and straightening unhardened metal.

A ball peen hammer has a rounded half-ball, or peen, instead of a claw opposite its striking surface. A mallet has a rubber, wood, plastic or rawhide head to drive chisels and to hammer joints together. It is ideal for brass and jewelry work, and automobile projects where a metal hammer could damage the surface.

There are dozens of other specialty hammers of various sizes. Sledge hammers, with handles of from 14 to 36 inches, are for heavy jobs where great striking force is required. They can weigh from 2 to 20 pounds.

Hand-drilling hammers, weighing 2 to 4 pounds, are small sledge hammers with short handles and are recommended for pounding hardened nails into concrete or with tools that drive nails and pins into concrete, brick and other masonry. Some of the other specialty hammers include those for riveting, metalworking, scaling and chipping masonry, finishing welding beads, etc.

Safety goggles should always be worn when using hammers against equally hard or harder objects. This is true when using any kind of striking tool. When striking another tool with a hammer, such as a cold chisel, punch, etc., the striking face of the hammer should have approximately twice the area of the struck tool.

Other safety precautions: Never strike with the side of the hammer. Never strike one hammer with another. Never use a hammer with a loose or damaged handle. Inspect the hammer often. Discard it when the face shows excessive wear, mushrooming, dents, chips, cracks or anything that doesn't seem to be the way it was when the hammer was new.

In checking the hammer, be especially careful about looking at the edges of the face; chipping may occur.

-Do-it-yourselfers will find much helpful information on a variety of subjects in Andy Lang's handbook, "Practical Home Repairs," which can be obtained by sending $2 to in care of the Deseret News at Box 5, Teaneck, NJ 076660).