Entry doors set square, sealed and weather-tight.

A straightforward pre-hung entry door replacement is typically a single-day job. It takes longer if the opening has water damage at the sill or the framing is out of square, both of which are common on older homes and both of which need correcting before the new unit goes in.

A sill pan is a waterproof tray under the door threshold that catches any water that gets past the door and directs it back outside. Without one, water that reaches the sill soaks into the subfloor and rim joist, which is why so many entry doors on older homes sit above soft, rotted framing.

It is usually worth doing together, because the storm door mounts to the same frame and fitting both at once avoids doing the same work twice. On a south or west-facing entry, be aware that a storm door over a dark insulated door can trap enough heat to affect the finish on some doors.

Yes, though swapping between sliding and hinged styles usually means modifying the rough opening, which changes the scope. Header sizing, flashing and interior finish all have to be addressed. If the existing opening stays, the change is much simpler.

Usually seasonal movement in the framing or the door slab, or a foundation that shifts slightly with frost. Wood doors also swell with humidity. If the sticking is getting worse year over year rather than cycling, that points to a structural issue worth looking at before adjusting the door.