With the forum upgrade it became difficult to embed youtube videos, now it has been made much easier.
To insert a youtube video into a post, first go to youtube and find the video that you want to post-
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In the address bar is the url of the video, highlight the url using the mouse by left clicking in the address bar, it will turn blue as in the picture above. Copy this line and then paste it into your new post-
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Now highlight the text by pressing left click on the mouse and moving it over the line of code.
You need to input the URL of the video you want to show, the url you have tried to use is what happens when you watch a featured/related video, so you have to remove the &feature=related for it to work