Avengers 4: 10 Theories From The Endgame Trailer

At the end of Ant-Man and the Wasp, Scott Lang shrinks down into the Quantum Realm with the help of a shrunken Quantum Tunnel located in the back of the X-Con Security Consultants van. Once he shrinks down, Hope, Janet, and Hank are all dusted by Thanos' snap, leaving Scott to languish in the Quantum Realm.

However, the Endgame trailer ends with a decidedly normal-sized Scott Lang pulling up in the X-Con van to Avengers HQ and asking to be let in. Considering that nearly all of the footage from this trailer seems to be from the first act of the film, it's a solid bet that Scott will either break free from the Quantum Realm very early in the film or that he'll just already be out by the time the film starts.

However, the Quantum Realm and his time spent within it will almost certainly be key to the plot. As Janet Van Dyne tells Scott at the end of Ant-Man and the Wasp, the Quantum Realm contains "time vortexes" that distort time and space. This means that Scott could have spent years in there and while only days passed in the real world, leaving lots of time for him to learn about it.

But even more importantly, Scott still has the Quantum Tunnel with him in the van in this trailer, meaning that there's a strong chance that these "time vortexes" are how the team plan to undo what Thanos has done with time travel(much more on that later).