You must copy the 'Embed code" from Youtube or else...

1.Click on edit page

2. Click on  "Source"; now you have the html format; choose the place where you paste the "Embed code". In the "embed code" we can change the size of video (proportionaly)!!! 

3. Click on Source; now you have the View format; now you can easy center the video, or you can drag and drop to change the place in page.

Similar we can insert a video in Forum Page:

 

From Mihaela Git (written in forum tread Tutorials)