Board Thread:Spoilers!/@comment-24844464-20140506132033/@comment-8345449-20140509103820

Maybe they created an alternate timeline but the timeline they came from still exists. So now there are two timelines that branch off from each other from the point when Hook and Emma travel to.

Now, if Emma did something so that she no longer exists in Timeline 2, even if she goes back to the same point of time in Timeline 2, where she left in Timeline 1, nobody would know who she is (except for Timeline 1 Hook), Storybrooke may not exist, and of course, there is no Henry.

In order to fix that, Emma has to make it such that Timeline 2 becomes very similar to Timeline 1. Everything with Zelena must also happen in Timeline 2 and the time travel portal must be opened in Timeline 2 as well. In that case, when Timeline 2's Emma and Hook get dragged into the portal, Timeline 1's Emma and Hook can come in to replace them.

Of course, it also means that Timeline 2's Emma and Hook wiill create a new Timeline 3. And Timeline 2's Emma and Hook have to follow the same steps and replace Timeline 3's Emma and Hook, who will then create a Timeline 4 and so on and so forth; until finally some Timeline's Emma and Hook fail to fix events properly and no time travel portals and new timelines are created...