Tanjiro doesnt know this because the 11th style is an original attack created by Giyu Tomioka but somewhere in the episode they said that every user can be able to have its own unique extra form.
The Eleventh Form is an original technique created by Tomioka, which is also referred to as Dead Calm. While there are ten known and established forms, the Eleventh is purely Tomioka's addition, allowing him to negate the attacks of the enemy by ceasing his own movements.
It seems that Tanjiro still has a long way to go to master the Water Breathing technique. Giyu, on the other hand, as the current Water Hashira, has complete mastery over all 10 forms of Water breathing, as well as an 11th form that he created himself.
While on the mountain, Striking Tide was the first form Tanjiro used. It uses flowing movements, allowing its user to perform multiple consecutive strikes to overpower their opponents. Using this form, he was able to take down two demons, an impressive feat considering how little experience he had up until that point.
Tanjiro Kamado, the series' protagonist, has mastered all ten forms of the Water Breathing technique, each of them equally deadly and effective depending on the situation and enemy.
Initially, Tanjiro's accuracy and strength were not up to the mark, which is why Muzan was able to avoid his attacks. However, Tanjiro accurately performed the thirteenth form and inflicted a ton of damage on the Demon King.
The Thirteenth Form of the Sun Breathing Style is unique in the sense that it doesn't really have a name of its own. Rather, it's a combination of all the twelve forms present in this style, used in succession to maximize their power. This move was devised by Tanjiro solely to defeat Muzan.
Tanjiro certainly did a lot of good with his Water Breathing techniques, but his ability to use Sun Breathing both gives him an edge in battle and allows him to stand out among the incredibly powerful Hashira he fights alongside.
The only known Demon Slayer who is capable enough to have two Breathing Styles at his disposal is Tanjiro Kamado. He initially started without any knowledge of Breathing Style and had no prior training in swordsmanship.
In the memories, Tanjiro observes Yoriichi performing 12 of the Sun Breathing formations, which help him perfect his usage of the technique. It is thanks to Sumiyoshi that the movements are passed down through the generations and eventually become known as the Hinokami Kagura.
Well, in the manga, does master the Sun Breathing style. Apparently, Tanjiro sees Yorrichi Tsugikuni, the creator of Sun Breathing, performing all forms of the breathing style. Interestingly, Tanjiro sees Yorrichi using the technique through Sumiyoshi's memories, an ancestor of the Kamado family.
It is possible for a swordsman to use more than one Breathing Style technique at a time, allowing the user to take advantage of the advantageous aspects of both techniques. Tanjiro Kamado, for example, can use both the Water Breathing and Sun Breathing styles simultaneously.
Also tanjiro can create as many forms as he wants if he can get stronger. the 13th form is the combination of all 12 forms of the sun breathing -_-
Eleventh Form: Dead Calm ( 拾 じゅう 壱 いち ノ 型 かた 凪 なぎ , Jū Ichi no kata: Nagi?) - A form of his own personal creation, the user ceases all body movements and enters a state of complete tranquility, deflecting, blocking and cutting any incoming attacks with imperceptible speed with his blade.
Of course, Nezuko never consumed any human, so Daki initially thought that she wouldn't be able to regenerate. However, Nezuko quickly restores her body parts, which obviously comes as a shock for Daki. After that, Nezuko transforms into her Berserk Demon form, which is also called her "Awakened Form."
7/11 Fifth Form: Blessed Rain After The Drought
The fifth form accomplished exactly that. Called the Blessed Rain After the Drought, it's an attack meant to decapitate enemies without causing any pain, which allows enemies who have surrendered to die a graceful death without feeling any unnecessary pain.
When Haganezuka sees Tanjiro's reddish hair, he becomes hopeful that he will get to see a bright red Nichirin sword. However, once Tanjiro unsheathes his weapon, the blade turns a dark black, to the concern of Urokodaki and the anger of Haganezuka.
In the far chapters of the Demon Slayer manga, Tanjiro is actually turned into a demon by Muzan Kibutsuji. Muzan does this to continue his demon legacy and live in this world even after being dead. He chooses Tanjiro over everyone else as he considers him the perfect one to carry on his legacy.
This Breathing Style is solely used by Michikatsu, who was one of the first Demon Slayers who utilized Total Concentration Breathing. Even after he became a demon, he continued to use it.
That being said, Tanjiro's rank is unknown, and his rank was last revealed in the Entertainment District arc. In that arc, they were at Kanoe, which is fourth from the bottom. Since Kiriya Ubuyashiki announced the disbanding of the organization, Tanjiro doesn't get the chance to become a Hashira.
The last user is Tanjiro Kamado, who learned the Breathing Style from his father, Tanjuro Kamado. Other people can learn this technique if taught by a Kamado descendant.
After returning to his human form, Tanjiro loses the ability to see from his right eye, the iris of which is now a dull pink-gray color and the pupil black rather than its original white.
Even 400 years later, Muzan's fear of Yoriichi remained fully intact, as simply laying eyes on Tanjiro's hanafuda earrings caused him to recall their clash.
The black blade represents the Sun and is surrounded by prejudice and misconceptions, as historically every demon slayer who held it lived a short life. This could mean that Tanjirou is destined to break the pattern and set a new legend for black Nichirin blades in the Demon Slayer universe.
However, Tanjiro is not a descendant of Yoriichi Tsukiguni. Tanjiro is the descendant of someone who knew Yoriichi personally and was a good friend of his as well. Sumiyoshi was rescued by Yoriichi when a demon threatened his life. Since then Yoriichi often met him and they became close acquaintances.