This movie is just amazing in all ways. Scenography, history and the special effects had been the best in the whole Saga.
This is in my opinion the best movie from Avengers, but still remaining several unanswered questions for those who don't know to much about Marvel's universe, it can make them think that many scenes were unnecessary. One of this scenes begins when Star Lord slaps Thanos preventing the gauntlet was taken away from his hand just losing the opportunity to defeat him, which makes Dr. Strange give Thanos the Time's Stone and making us think:
Why he just didn't make the time go back and prevents Star Lord to committing such stupidity?
Theory 1:
In Dr. Strange's movie we saw him doing a loop time to "negotiate" with Dormmam, in this way he managed to chase him away from the earth forever. However the dimension in which Dormammu was found is one in which there is no time and therefore can leave the loop, if Dr. Strange had used the gem to trap thanos in the loop time would have created an eternal cycle of time throughout the universe that would affect everyone and cause a natural disorder of time and space.
Theory 2:
The gems can't fight between them because none is more powerful than the other, as explained in the comics (this is the reason why Thanos did not use the infinity gauntlet against Loki but that he threatened to kill Thor if he did not deliver the space gem), so if Dr. Strange tried to use the Time's gem this wouldn't work and would be in a fight of a magician in the process of conversion to Supreme Sorcerer against a millennial and immortal Titan.
Theory 3:
"The only way to beat Thanos is to make him believe he won"
We know that Dr. strange saw 14.500.000 possibilities of alternative futures and only in one of them they won, the most certain thing is that he knew that Thanos wouldn't stop until he achieved his goal, thich is, in all future Thanos would exterminate half of the population of the universe and for this reason one of the possibilities included changing the gem for Iron Man's life since he knew that Tony Stark would play an important role in that future in which they would win, this reaffirms even more at the end when Dr. Strange tells him "Tony, it was the only way."
These are the theories that I've been able to conclude until now.
Do you have any different one?
Share them in the comments.
All images are from avengers Infinity War movie
most of them are reedited.