Eren is put at the bottom of a dry well and will try to induce a transformation when given a signal. He repeatedly bites his hands, hoping the pain will cause the change.
He bites his hand in order to cause himself pain to transform. To transform into a titan, you need to cause pain to yourself and have a goal in mind.
However, when him, Mikasa, and Armin were about to be crushed by a canonball, Eren mostly had an instinctual reaction. Since Grisha and Frieda (was that her name?) Both knew how to turn into a titan, it came to him naturally. One of them probably bit their hand like he does, too.
A Titan Shifter needs to draw blood in order to transform, and some, like Eren, bite into their hands. This can be a painful experience, but Shifters do not need to worry about these injuries because they will heal extremely quickly. When someone inherits one of the Nine Titans, they gain the ability to regenerate.
Eren needed time to discuss his plans with Zeke and the others. And the only way to buy more time was by injuring himself and getting himself admitted to the hospital by presenting himself as a war soldier. This way, he also averted himself from being the suspected Eldians intruding on Marley's land.
Why Eren Says He Hates Mikasa in Attack on Titan. Eren hates Mikasa's supposed lack of free will and claims he always hated her for having no freedom. Eren points to Mikasa's headaches as proof her Ackerman bloodline is all that's keeping them together.
A Titan, with Eren in its mouth, bites down and severs his left arm and then eats him.
Because it is impossible for anyone to surpass the Founder, each person who gained the power of the Titans was fated with the "Curse of Ymir" (ユミルの呪い Yumiru no Noroi?), which limited their remaining lifespan to only 13 years after first acquiring it.
Witnessing his mother's death was traumatic enough, but seeing countless allies die in front of him was a major factor in Eren's fall to the dark side. Several of his friends died when the Titans invaded Trost.
Or, to quote the wiki: If an individual with the power of the Titans dies before it can be transferred, the power will be passed on to the first Baby among the Subjects of Ymir, who is born directly after the said inheritor's death, regardless of distance or relation to the previous inheritor.
With this in mind, Eren's laughter becomes something much more ominous. It becomes a sign of not only despair at losing Sasha but his resignation to the future. Sasha's death may have been the thing that drove Eren over the edge, forcing him to confront the absurdity of resisting his fate.
When Season-3-Eren kissed Historia's hand, he saw Grisha's memory of his slaughter of the Reiss family, which Grisha did on Future Eren's orders, with Season-4-Eren present via Zeke's Founding Titan powers.
If you are taken out of your Titan and then injured, you have to wait until you heal to transform a second time, like what happened with Eren's arms in season 2 and Zeke's legs in season 4. It's like Levi said, “if your body is too busy healing, you don't have time to transform again” after he beat Zeke the first time.
Eren's anger was hatred for the Marley Kingdom. After learning the truth about the Titan and who the real enemy (who in the story is Marley), Eren's anger has intensified. Eren hates Marley because they sees himself and the nation as a demon.
His intense emotional output shows he definitely suffers a lot from his Titan powers, but the pain inflicted on him doesn't seem to have much effect. A guy who willingly gnaws on his palm regularly can get used to a bit of pain, so Eren doesn't spend much time languishing over physical discomfort.
According to AOT's website, Eren Yeager is 19 years old in this final season. Mikasa Ackerman is also 19. An important thing that many AOT fans might already know is that the Ackermans meaning Levi and Mikasa are special, they do not undergo ageing like others do.
On one hand, Eren fights for the true freedom of his people, but on the other, he believes his bloody methods justify his cause. This dichotomy makes him one of the best-written anime characters, as he can often be considered a brave hero or a devious villain.
It was because of his future-seeing abilities that most of the story transpired the way it did, which makes Eren seem even eviler than how he was initially presented. Eren wasn't always an evil character, but there are plenty of hints sprinkled throughout the series regarding what he was destined to become.
Eren didn't turn “evil” per se so much as he saw that the world did not have any kind of clemency toward his race. It's because he saw that the entire world hated him and his kind that the only solution he could arrive at was to kill the entire world.
They do seem to become less active at night or if deprived of sunlight, and so it may be possible they draw their primary source of energy from the sun.
The legend of the first Titan, Ymir Fritz, details how the Titans' founder lived for a mere 13 years, and thereafter, no Titan could surpass that limit. Grisha himself was said to be proof of these limitations, feeding himself to Eren after having been in possession of the Attack Titan for 13 years.
4 Perk: Pure Titans Practically Live Forever
When someone is turned into a Pure Titan, they're stuck in their current age until they're turned into a Shifter. In effect, Pure Titans are immortal.
Well, that's because a larger part of Eren's plan required him to not be discovered at Marley. In order to perfect that disguise, he gouged out one of his eyes and cut off his left leg so he could play the part of someone else.
Officially, Isayama has said that the reason for Eren's growing hair was that he hasn't been concerned with the way he looks lately. It didn't mean misery, just that his focus geared to his internal thoughts and the actions going on around him (and there was a lot!).
Eren's death and Ymir's liberation by Mikasa caused every Titan body turning to dust. Those that had been transformed were restored back to human form.