Can you use speedlite SB-800 on auto mode?

Hi
i'm thinking of buying sb 800 for my nikon d3100. I was just wondering if i can use it on auto mode not just manual.
Is it suitable for my nikon?
anything important i should know before i buy it?
thank you

Added (1). I mean if the flash its TTL(i think is called like tha)(i don't have to set anything the flash will do all the work?)

When you use a speedlight, it will flash under the same conditions that the internal flash on your D3100 will fire. You probably already know then that when your camera is in auto mode, you have no control over when the flash will fire or not. The flash simply pops up when it is needed.

You will have that same functionality/limitation with a speedlight with the camera in Auto mode.

Learn to use PSAM modes; this will allow you to become more creative, and allow you to use fill-flash and other techniques. Auto modes on a DSLR are only there as "training wheels" to get you into a DSLR.

Using a fill flash - even in the daylight, will allow you to compensate for shadows, or to properly expose both interior and exterior segments of a photo (say you are shooting through a window), or any other situation when the lighting is good enough for a proper exposure, but not good enough to keep harsh shadows from your photo.

Here is an example:

The first like shows a photo taken without a flash with the camera metering the inside. Notice that the foreground is properly exposed, but the background is not. This is a result of low dynamic range and is apparent in all cameras:

The second photo, also taken without a flash, but this time the camera is metering the background. And the opposite effect is the result. The background is properly exposed but the foreground is under exposed. Again, it is a result of low dynamic range.

And this photo was taken with a fill flash. The flash properly exposed the foreground, but was not strong enough to over expose the background. To take this photo, the camera and flash were both put into manual exposure mode. The flash was set to properly expose the foreground, and the camera was setup to properly expose the background.

This result would have been impossible with the camera in Auto mode.

Yes.

No problem. When flash is in Auto mode, you set your camera to Manual mode at maximum shutter speed (1/500s? Verify in manual) then check flash for recommended aperture size for distance range you want to use.

Check your camera's manual if the SB-800 is included in TTL compatibility. TTL is different from Auto. With TTL, you set flash to TTL and that's it. The camera takes care of everything.

That flash will work just fine in any mode that the flash or the camera supports. Meaning yes, in iTTL mode, the flash and camera do all the work.