The password phrase for handling a common household problem - a stuck window - is "be gentle."
If you aren't gentle in unsticking the window, you will wind up with another common household problem: what to do about the ugly mars around the window sash and molding where you finally got the window to move again?Gouges in that area are the usual result of getting the window loose the wrong way. The wrong way is using the blade of a screwdriver instead of a wide-bladed putty knife.
The screwdriver blade is usually too thick and too narrow for the prying action. You should use the wide-bladed putty knife or a special serrated tool made especially for the purpose. Even then, the prying must be done carefully - in other words, gently.
The usual reason for a stuck window is a paint seal, which needs only to be broken to get things in order again. Examine around the sash - with a flashlight, if necessary - and see if you can see where the seal exists.
Sometimes the window will move after the broken-seal treatment is used on a single side, sometimes you will have to do it on two, three or four sides. If the cause is a warped stop molding, the same treatment will put things in order.
In both cases, you need some post-solution action. If paint caused the problem, sand lightly and lubricate. If warping was the culprit, straighten out the molding or use a new strip. But if high humidity conditions brought about the warpage, the same thing will occur again at a later time unless the condition is fixed.
For instance, it doesn't do much good to remove the mildew, since it will return soon.
As in all do-it-yourself repair problems, you sometimes will face an obstacle that can't be hurdled easily.
Suppose breaking the paint seal does not work - or suppose there is no paint seal, yet the sash remains stuck - what do you do?
Get a wooden block of some kind and tap it alongside the grooves of the sash. Do it very carefully to avoid breaking the pane or anything else.
In an especially stubborn case, you may have to work on the window from the outside, again using the wide-bladed putty knife technique. Insert the knife blade between the bottom of the sash and the top of the sill and, if necessary, wherever sticking persists.
When none of your efforts bears fruit, you may have to take the somewhat drastic action of removing the sash from the frame. This is a bit sticky if you haven't done it previously, so get some advice from a knowledgeable neighbor or the local hardware dealer.
Better yet, get one of the handyman publications that give detailed instructions on how this is done and how to determine if the sash cords need replacement.
If so, consider the possibility of replacing them with more modern spring lift sashes or balances. Most of them are a lot easier to install than the old-fashioned sash cord. They come with complete instructions.
One of the possible annoyances with double-hung windows (those that slide up and down in parallel tracks) is that, when they become very old, the latch fails to hold. This is because the hook on the upper sash is no longer in alignment with the lock.
All that needs to be done is to unscrew the hook and reset it so that it locks properly. The holes that have been left in the sash can be filled with wood putty or plastic wood.
When you keep a window open and cannot use the latch, creating a security risk, try buying the little gadgets that keep the window open part way but prevent it from being opened any farther. This provides air without a loss of security. They are easy to attach.