The basics of time travel go like this

There are 3 possiblities -

1. each time you go in the past, you alter something your future is altered as well. This theory never works because - if you kill yourself, you wouldnt exist but how did you time travel to kill yourself in the first place? A paradox. So this theory doesnt hold.

2. Each time you change something in the past - something happens to self correct itself. For example, if u kill baby hitler, some other kid grows up to become hitler.

3. Each time u change the past - you create a new timeline. This is where if you kill yourself, you create a new timeline where you died but you still exist in the your original timeline.

Endgame uses a combination of theories 2 and 3.

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First before going into the timelines and time travel of Endgame, we have to establish some ground rules.

*Ground Zero: The movie cannot be explained without multiple timelines however hard you try. Because, like Professor Hulk says, you cannot simply alter the past. This means either one of 2 things - multiple timelines or single self fixing timeline. **But self fixing timeline is not possible here since when an infinity stone is removed, the timeline cannot self fix itself. A new infinity stone cannot be created to replace the lost one. So, multiple timelines it is.*

*Ground Rule 1*: Only removing an infinity stone from a particular time creates a new timeline (like Hulk and the ancient one discuss)

*Ground Rule 2*: The main timeline will be OG(original) timeline, where things wobble and are flexible because of minor changes but a new timeline is not created for each small change.

*"The infinity stones create what you experience as the flow of time. Remove one of the stones and that flow splits" - exact words used by The Ancient One to Bruce Banner. So we are confident that Ground Rules 1 and 2 are correct.*

Now, there are 6 new timelines created in total, in addition to the OG timeline. One where each stone is stolen.

1.  Soul stone - Hawkeye returns back to the original timeline. A new one was branched off the moment Natasha dies in the original timeline, since Hawkeye now has the soul stone. This is timeline 1. *Since Natasha dies in the OG timeline, she cannot be brought back.*

1.  Time stone - Professor Hulk gets the time stone from the ancient one and returns. Now a new timeline is created without the time stone, since Hulk has returned to the OG timeline. This is timeline 2.

1.  Mind Stone,
1. Space stone - These two are tricky because a lot of shit happens here. *Loki escapes with the space stone BUT according to Ground Rule 1 and 2, a new timeline is not created yet. Because he doesnt jump to another timeline. He is still in the same timeline. This timeline will somehow fix itself, leading upto Infinity War. Maybe Cap finds Loki and puts him back where he disappears (OR) since Loki anyway escapes at the end of Thor 2 and space stone ends up with Thanos in another way. But, this also gives Loki all the time in the world to have his own side adventure in the Loki Disney+ series. **This all happens before Ant Man returns back with the mind stone, so this is key that they are still in the OG timeline.*

 Now Ant Man returns with the mind stone - This creates Timeline 3 without the mind stone.

 CA and Iron Man go to 1970 - they are still in the original timeline. When they steal the Space Stone - this creates timeline 4 - without the space stone.

5. Reality stone - Thor and Rocket steal the reality stone - this creates timeline 5 - without the reality stone.

6. Finally, Power stone - This is where it gets a bit confusing. Now, the key here is, War Machine escapes with the Power stone before OG Nebula. This means - a new timeline 6 is created without the power stone. *This means, OG Nebula is now in Timeline 6. Now, the Thanos from this timeline 6, uses the pym particles of OG Nebula to jump to the OG Timeline.*

*Now, the OG timeline's Thanos did cause Infinity War and is now dead. Only the Thanos from 2014 Timeline 6 has now come to the OG timeline.*

All the new 6 timelines + OG timeline exist currently, simultaneously, with one another.

The small changes(like Tony - Howard interactions) are all self correcting in the original timeline.

Now, Tony destroys Timeline 6's Thanos and dies. So Timeline 6 never has an Infinity War.

Finally Cap, returns all the 6 stones to the OG timeline immediately after they are taken, thereby closing the branched timelines off - Like Hulk and Ancient one discussed. Then Thanos in the OG timeline proceeds to collect them.

Whether Cap returns to the OG timeline and lives his life - then normally ages to reach 2023 OR whether Cap lived his life in one of the 6 new timelines and then finally returned a stone, closed off the timeline and came back to the OG timeline - is open to speculation.

Timeline 6's Gamora sticks around here in the OG timeline since she was not snapped. She wouldnt be affected even if Timeline 6 is closed off by Cap. Gamora is also not included in Tony's snap to kill Thanos and his army.