I can only pause and continue video, but how do I fast forward?
I have tried tapping and swipe on timeline but it doesn't work that way?
The phone's built-in video player, which is used to play videos on IE and other apps, doesn't have a seek mode like you'd expect. The Next and Previous are actually used to skip forward and backwards within a clip.
The only problem is that the buttons appear to skip a fixed amount of time, so if you're watching a short 15 second clip from YouTube, the Next button will most likely skip over the while clip. With longer clip of a few minutes, skipping forward and back seems to work well.
If you watch a lot of YouTube videos, I would recommend Metrotube, which is a free YouTube client for Windows Phone. It features a custom player which has the ability to seek within a clip using the on-screen controls.