Well, to build tension in this fanfiction, the author should focus on Ponyboy's internal thoughts. Let the readers know what he's thinking, like his fears and his desperate attempts to figure out a way to escape. The use of sound can also be important. Maybe the sound of footsteps approaching him quietly in the dark. And don't forget to make the antagonists seem really menacing, like they have a personal vendetta against Ponyboy.
One way is through detailed descriptions. For example, describe the look in the eyes of the attackers, how their muscles tense before they make a move. Another way is by using short, choppy sentences to mimic the quickening of Ponyboy's heartbeat. Also, having the surroundings play a role, like a dark alley with no way out can really build the tension.
One possible reason could be that he's a Greaser. The Socs might see him as an easy target just because of the group he belongs to. They might want to show their dominance over the Greasers.
Tension can be built by introducing unexpected twists and turns. Maybe a trusted ally betrays the protagonist or a seemingly easy solution turns out to be a trap. Additionally, ratcheting up the stakes for the characters makes the story more tense.
You can start by creating a sense of uncertainty. Make readers wonder what's going to happen next. Also, build up to important events slowly to increase anticipation.
You can build tension by creating time pressure for the characters or introducing unexpected obstacles.
One way is to introduce time pressure or deadlines for the characters. When they have to act quickly, it naturally creates tension.
One way is to create a sense of time pressure. Make the characters have a limited amount of time to achieve their goal.
In this fanfiction, it might be due to his poor living conditions. He doesn't always have the best food or a warm, clean place to stay. His body could be weakened from not getting enough nutrition, which makes him more likely to get sick. Also, the emotional turmoil he goes through with losing friends and dealing with the Socs could take a toll on his immune system.
Build tension through setting. A dark, spooky forest or an abandoned, dilapidated house can create an atmosphere of unease. If your characters are in such a setting, the tension will be palpable. You can also use pacing. Slow down the narrative when something crucial is about to happen, like a character making a life - changing decision. This draws out the moment and increases the tension.
The author can build tension by constantly threatening to expose Sakura's true identity. For example, having a character get suspicious and start to investigate. Another way is to create situations where Sakura has to choose between maintaining her disguise and doing what's right. If she reveals herself, she might face consequences, but if she doesn't, something bad could happen.
One way to build tension is through the use of pacing. Don't rush the scares. Let the reader sit in the unease for a while. You can also use the power of suggestion. Instead of showing the horror outright, hint at it. For instance, describe a shadow that seems to have a life of its own, but don't immediately show what it is. This makes the reader's imagination run wild, which is often scarier than actually seeing the horror. And make the characters' decisions matter. If they make a wrong choice, it should lead to more danger and more tension.