Doctor strange owns the time stone which give him a power to play with the time. His journey begin as a doctor strange with a dramatic start In New York City, Stephen Strange, a rich, popular, and arrogant neurosurgeon, badly injures his hands in a car accident, leaving him unable to operate. Fellow surgeon and old lover Christine Palmer tries to help him move on, but Strange ineffective pursues experimental surgeries to recover his hands, almost bankrupting himself.
Strange finds out about Jonathan Pangborn, a paraplegic who mysteriously got his legs healed. Pangborn mentioned Strange to Kamar-Taj, where he is taken in by Mordo, a sorcerer under the Ancient One. The Ancient One demonstrates her power to Strange, revealing the astral plane and other dimensions such as the Mirror Dimension. Ancient help strange to become doctor strange.if you want to know more about Doctor Strange click on wikipedia link
14,000,605 possible outcomes has be seen by doctor strange which shows that how hard and difficult it is to kill the thanos and save the world. In the Avenger infinity war they showed Strange using his power to work out how he can defeat the thanos by going back and forward in time, while he was doing this his head was badly shaking and when he saw all the outcomes there was sad look on his face which explain that he has seen his on death. Doctor could have kill the thanos when, thanos garbed by the guardian of the galaxy, toney, peter and strange himself by creating lop or cutting his hand as strange did in the beginning of the movie.
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But that’s not happened and he has also seen this idea not working. Strange could have seen more possible futures but he stopped when he realized it that its not all about him h had to lose in order to win the endgame.
It also possible that strange has played with the time and moved everything into future made thaons look like all going into his favor and when the right time comes,
he has all the power along with other super heroes, the one moment among 14,000,605 version of the future he got his chance to turn the table into his side and win the game and kill the thaons. Save the humanity fix everything as nothing has happened. It could also be possible that not the Toney but the Thor can save the world from thanos. Thor has more powerful than any other super hero so far and in the Avenger Infinity war Thor has almost kill the thanos when he threw his new weapon into thanos chest
“Thanos said you should have aimed the head instead the heart” can give a clue that strange reverse the time, Thor took his chance to kill the thanos.
There are many theories and many explanations are coming out day by day as the release date is coming close. We all can predict what can happen but only can find out the actual mystery on 24th of April.