1-When would you use external flash?

1. Poor lighting.
2. A fast-moving subject.
3. Eliminating or adding shadows.
4. Night photography.

2-Do you need to meter the light if you use flash?

If you are using a flash, getting a light meter that can give you an accurate flashlight reading is essential.

3-What does flash synchronization mean?

flash synchronization or flash sync is the synchronizing the firing of a photographic flash  with the opening of the shutter admitting light to photographic film or electronic image sensor.

4-What does ETTL mean?

It means Evaluative through-the-lens and is a type of auto flash exposure system used by Canon. E-TTL 

5-What will happen when you shoot faster than the camera sync speed?

By using a shutter speed at or below the camera’s sync speed, the entire sensor is exposed to the flash, resulting in proper exposure. Faster shutter speeds can cause dark bands or underexposed areas in the image.

6-Why do photographers bounce the flash and use a diffuser?

A flash diffuser is used to turn the spotlight sort of illumination to a more spread out environment filling light.

7– What is a slave?

Slave mode is mode that instructs a flash unit to monitor incoming light, and fire when it senses the light produced by another flash unit firing.

8-What should you do if your image comes out too dark or too light?

Too light either the aperture or shutter speed needs to go up.  And if there coming out dark you a fill flash.