When a bright window is in the background, use the flash

When photographing human subjects indoors and against the background of a bright window, turn the flash on. This is similar to shooting back-lit subjects outdoors or in the shade.

Bright window in the background spoils the exposure and the subject is under-exposed

When shooting indoors, your camera must be set so that the reflection of the flash does not enter the lens. If reflected light enters the lens, the image may be marred with red lines or white spots.

You can either locate the camera and subject so as to block any reflected light coming from the window, or at an angle so that the light from the flash is not reflected back towards the camera.

The camera was kept away from the reflected light and the flash was used

Compare:Understanding Digital Cameras/Influences of subject and background brightness

Compare:Understanding Digital Cameras/Indoor photography with and without flash