In the Australia versus Samoa game when Australia scored their first try ,the ref asked the video ref to check it out.The on field ref said 'Check if it has been knocked forwards'.The video ref said 'It definitely went backwards'.The video ref judged that it wasn't a forward pass but the video ref isn't allowed to judge on a forward pass.Neither the on field ref or the video ref used the phrase 'forward pass' but they did judge whether or not the pass went forwards.I found this very confusing.

How come a forward pass is not off side - surely the reciepient is front of the last person to touch the ball?

Depending on how far the ball travels it is perfectly possible to be behind the person passing and to catch the ball in front of the point of release.
This general topic has been done to death. Once again I refer everyone to the RFU video explaining how a ball can go backwards from the hand and still go forward relative to the ground. One further development to the topic is that with a Hawkeye like system it should be possible to factor in the speed of forward motion by the player to calculate if the forward movement of the ball was in line with a ball that had gone backwards from the hand. Then I thought for God's sake just get a grip.

They would only be offside if they were in from when the ball was passed. Similar to a kick, if you chip over the top and a chasing player gathers they are in front of the last person to touch the ball.

A knock on is forward in relation to the pitch.
A forward pass is in relation to the player.
Camera angles can't pick up on forward passes very well, but can on knock ons.

Saying that, I still think video ref should be able to judge forward passes. They have plenty of angles to judge from.

For me that first try was a clear knock forward. There was no 'pass' as he was just batting the ball, and IMO it clearly went forwards as the on field ref thought. Wouldn't have changed the result but a bad early call nonetheless.