Attack on titan is one of the most successful anime's, but you might wonder about some plots and twists. I'm here to answer as many questions as possible. Request in the comments!
The majority of titans are known as 'mindless titans'. A mindless titan is basically the nickname given to titans who cannot revert back to their human form. The reason I'm saying that a mindless titan cannot 'revert back to their human form' is because mindless titans are actually humans.
As a severe punishment for the Eldians who break the rules Marley set, the Eldians are transorted to the island of Paradis, the place where the Walls are.
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Once transported to the dock at Paradis they climb a wall similar to that of Wall Maria, Rose and Sina. At the top of the wall, Marleyan soldiers would inject the Eldians with a serum made from so far unknown ingredients. This turned the Eldians into mindless titans. Before the transform though, they are pushed off the wall and onto the island of Paradis.
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At 2:26 in the video below you can see Ymir's experience being turned into a titan.
The only way a mindless titan can be human again is if they eat a titan shifter and ingest their spinal fluid. Even if they succeed in doing so, they cannot be fully human, but will become a titan shifter themselves. For example, if a mindless titan ate the titan shifter for the Beast Titan they will turn into a titsn shifter, swapping between human and Beast Titan. Same goes for the rest of the main titans.
Sadly, after 13 years the titan shifter would die, due to the curse of Ymir which was covered in the last chapter.
