This story provides solutions to the most common door problems.
To get that satisfying "thunk" when a door closes -- rather than rattles, squeaks, or scrapes -- there must be an even gap between the door and the jamb all around; the hinges should be flush with the jamb and move freely; and the stop and the strike plate should be correctly aligned, so the door's latch easily clicks into the hole in the strike plate when the door closes.
The door itself may be solid-core, hollow-core, or made of panels. Hinges are attached to the jamb, which is attached to the house's framing on the sides and above. An interior door usually has two hinges and a heavier exterior door usually has three. Usually there is a gap between the jamb and the framing, which is filled with shims positioned near the nails.
Stop molding is positioned so the door bumps against it when closed. If the stop is too tight, the door will be difficult to close; if it is too loose, the door will rattle. On the latch side of the jamb, a strike plate is positioned over a hole in the jamb; the door's latch bolt engages a hole in the strike plate to latch the door. On an exterior door there is often a dead-bolt lock as well. The holes and strike plate must be correctly positioned for the door to close properly.
Hinges are attached to the other edge of the door with screws. They grab effectively only in solid wood (not particleboard). Long screws can be used if the door is solid wood, but shorter screws are used for a hollow-core door or a solid-core door with a particleboard core.
A squeaky hinge may only need a squirt of the right lubricant. If you see rust, first use penetrating lubricant to free rusted parts. Then apply powdered graphite or silicone lubricant for a longer-lasting solution. Also use lubricants to free a balky latch bolt.
On the jamb leaf of a hinge, long screws are effective if they can reach house framing. Where the screws would go into drywall, shorter screws are just as good.
If a strike plate is recessed (as often happens when the doorway is painted several times), remove the screws, pull it out, and make cardboard shims to fit in the mortise. Use as many shims as needed to bring the strike flush with the jamb.
If a door binds and a loose hinge is not the problem, close it until it just touches the jamb (don't force it closed) and use a pencil to scribe a line where the door needs to be trimmed. The ideal is a 1/8-inch gap between the door and jamb at all points. Mark both sides of the door.
Adjust a plane so the blade barely protrudes beyond the base. Test on a scrap piece of wood; the plane should easily produce very thin shavings. Readjust as needed. You can use a shaping tool, but the resulting edge will require sanding to make it smooth.
Apply wood glue to the top of the door. Attach the extension piece by drilling pilot holes and driving finishing nails or screws. Plane or power-sand the piece on both sides so it is flush with the door. Fill the joint with wood putty and sand again before painting.