Alright, i think i've quoted too many people who use the youtube tags incorrectly, linking to a white youtube video, that obviously, doesn't work. So here is the CORRECT way to link a youtube video.
You essentially, just copy what you have after the "V=" in the url, and place it inside the Youtube tags, which are [y t] and [/y t], without the spaces.
Say you have the link:
http://youtube.com/watch?v=123test321
Your Youtube, so that it would appear as a video on the site, will be:
[y t]123test321[/y t] Without the spaces
Another example of an actual video:
http://youtube.com/watch?v=6e19e9vg0Fs
Would be:
[y t]6e19e9vg0Fs[/y t] Without spaces
Which when used without the spaces, comes up as:
[yt]6e19e9vg0Fs[/yt]
You do NOT just post the link in the youtube tags:
[y t]http://youtube.com/watch?v=6e19e9vg0Fs[/y t]
This will show up as a white page
[yt]http://youtube.com/watch?v=6e19e9vg0Fs[/yt]
Edit:
Occasionally you will get a link with a lot of stuff in the link as well, just an example:
http://youtube.com/watch?v=6e19e9vg0Fs&randomlet
For this, you want to coppy what comes AFTER the "v=" but BEFORE the "&" symbol in the link.
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Hopefully this helps people when posting their youtube videos, so that it doesn't just post a blank youtube video
If you still have questions, or don't understand fully, i can clear things up for you.