Dog urinates in the house every time you leave him alone
This may be a separation issue. If it happens every time and if it happens whether you leave for five minutes or five hours, then speak to a dog trainer or behavior professional. Do not scold your dog for this; it will only make the problem worse.

Older dog (over ten years old, usually) urinates in the house after years of being housetrained
This may be a sign of aging or a Canine Cognitive Disorder. Either way, you should go to the veterinarian for a discussion and possibly medication. This is not your old dog’s fault – walk him more, love him more, but don’t scold.

Housetrained dog urinates in the house after surgery or when on medication
This is usually a result of the stress or medications. Be more attentive to when your dog needs to go outside, and perhaps keep him in his crate or confined space a bit more with regular trips to relieve himself. In a few days he typically should be back to normal. If he isn’t or if you are concerned, speak to your veterinarian.

Female dog urinates small amounts frequently, perhaps even in sleep
If you notice any uncontrollable dribbling of urine, whether in a sleeping area or when awake, you should contact your veterinarian for an exam. This may indicate a problem with lowered estrogen levels, as will sometimes occur in spayed females. Estrogen is important to help keep the muscles in the urinary tract firm and toned, and when these muscles become weak due to a lack of estrogen, the urine may dribble out.

Urine dribbling or involuntary urination (accidents in the house) may also be caused by a urinary tract infection, and blood in the urine may be evident. In either case, your veterinarian will want to get a urine specimen and may prescribe medication to help the situation.

Male dog urinates in house by lifting his leg, even just after a walk
This is leg-lifting or marking and has nothing to do with housetraining. This is generally an attitude problem more than anything else. The first step is to neuter your male dog, then speak to a dog trainer or behavior professional about getting this problem under control.

Dog urinates when you bend over to pet him or when he is nervous
This may be submissive urination. Young puppies do this with adult dogs as a polite or appeasing gesture and some continue to do this with humans. Scolding will only make the puddles bigger. Instead, try ignoring your dog when excited or anxious. You might try tossing a kibble off to the side to give him something else to think about.

If you suspect submissive urination, take the stress out of coming home. Don't acknowledge your dog until you get in and put your stuff down. Then walk to a tiled area – just in case – kneel and speak gently to your dog. Stroke the chest, rather than reaching over the head. With guests, wait to introduce your dog until they’re in and settled and things are quiet and calm. If this continues, get some professional advice from a dog trainer or behavior professional.