I've been trying during the last half an hour..
Weeeellllll...Okay, I guess it should be this way:
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You must have seen the message (by clicking the broken bridge), which I can't remember the exact words by the like:the bridge is damaged blablabla... then click on the barrel. It will say there's a hole on it, otherwise it would be a perfect boat. Click the barrel again(Maybe by the way I also click the planks). And the message 'I can repair the barrel with them' appears. Then the prompt says 'Now, I need to attach the planks'... So after this appeared, repeat what you've done for the last half an hour.:p