I just wanted to clear something up for myself. when I use the Flash Ettl in manual mode, it ask like the main liight source right? Only when you are in AV and Shutter does the flash act like fill flash?
Now if that is true, would it make more sense to shoot manual and manual flash?
how does the flash work in other are modes?
ETTL does not know if you want it for fill or as a main light. It just casts the right amount of light onto the image to give a good exposure on whatever you are metering on. For fill flash I usually dial in about 2/3 stops of negative FEC flash exposure compensation or it tends to wash out your subject.
Sometimes I shoot in Manual with the flash on ETTL and spot metering because I can, within reason, set both the shutter speed I want and the aperture I want and the flash will cover minor over or under exposure and give a good exposure on the subject, though the background may be underexposed.
The E-TTL flash behaves differently depending on what mode you are using.
In Program mode, the camera will meter the scene, but will set a shutter speed between 1/60 sec (that's the slowest speed it will use) and the max flash-sync speed for your camera (depending on model that could be 1/160th, 1/200th, or 1/250th.) This avoids using slow shutter speeds that might result in motion blur. But it also means the exposure might not be long enough to capture lighting in the background (so if that's what you want... you might want to use Av mode.)
The bevavior actually depends on a menu setting. Find a setting in your camera menu called "Flash Sync Speed in AV mode" (I don't know what camera model you have, so I can't tell you where to find it.)
This is the setting that determines how the flash will behave when shooting in Av.
If you set it to "Auto" (the factory default) then the camera is going to meter the scene as if you have no flash on the camera. It'll set the exposure for the scene. But it will add the flash to illuminate your subject. This is the mode that will use a slow shutter to capture the ambient lighting of the background.
HOWEVER... if your scene is dim, the "Auto" setting for "Flash Sync Speed in AV mode" could result in a very long exposure (it wont let the shutter use anything faster then the camera's max sync speed... which might be 1/160th, 1/200th, or 1/250th depending on camera model.) But it COULD allow the shutter speed to be as SLOW as 30 seconds. Obviously a 30 second exposure could result in a lot of blur and you may not want this.
So you can change this setting. On my 5D III, I have a choice for "1/200th - 1/60th" sec range (meaning we insist on keeping the shutter speed faster than 1/60th which is usually fast enough for hand-held photographry as long as the subject isn't moving and we aren't using a long focal length lens (especially if the lens doesn't have image stabilization).
And my other option on my camera model is "1/200th" - which insists that Av mode use 1/200th ... and no other speed. In other words, the Aperture is whatever I set, the shutter is fixed at 1/200th, and the flash will do whatever it has to do to the output power to guarantee a good exposure at that speed. That will generally mean there's extremely little chance of motion blur (you'd need a very fast moving subject to still have blur). BUT... it also means that in dim/dark situations the shutter wont be open long enough to collect much of the ambient light in the background and you may get the effect as though the only light in the room was from your flash and the background is almost completely black.
So "Av" mode isn't necessarily *just* a fill-flash mode... you get to control it with this menu setting. Look for it in your camera manual. It will be in the section on shooting with flash.
Somewhat similar to Av mode (except there's no secondary menu setting in Tv mode) the camera meters the scene and attempts to use an exposure that will capture the background.
If the aperture setting is blinking then it means that the camera is already at max aperture for your lens but wants even more light (in other words the background will be underexposed.) However it will still correctly illuminate your subject (it's just warning you that your shutter speed is too fast if you also wanted to capture ambient light for the background.)
Also... if you set a shutter speed faster than the max flash-sync speed, the camera will reduce your shutter sped to the max flash sync speed UNLESS your flash has High Speed Sync (HSS) support AND you have enabled the HSS option.
You set the exposure on the camera. The camera will set flash power to adequately illuiminate your subject using those settings. (Camera is manual, but flash is automatic because you are using E-TTL.)