1-When would you use external flash?

you would use flash when your lighting isn’t well.

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

yes

3-What does flash synchronization mean?

the fastest shutter speed in which camera can successfully progress the flash  

4-What does ETTL mean?

evaluative through the lens 

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

there could be dark area in your picture. 

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

to turn the spotlight sort of illumination.  

7– What is a slave?

 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?

adjust your exposure